potential climate-change impacts on the chesapeake bay

lable at ScienceDirect

Estuarine Coastal and Shelf Science 86 (2010) 1ndash20

Contents lists avai

Estuarine Coastal and Shelf Science

journal homepage wwwelsevier comlocateecss

Invited feature

Potential climate-change impacts on the Chesapeake Bay

Raymond G Najjar a Christopher R Pyke b Mary Beth Adams c Denise Breitburg d Carl Hershner eMichael Kemp f Robert Howarth g Margaret R Mulholland h Michael Paolisso i David Secor jKevin Sellner k Denice Wardrop l Robert Wood m

a Department of Meteorology The Pennsylvania State University University Park PA 16802 USAb US Green Building Council 2101 L St NW Suite 500 Washington DC 20037 USAc USDA Forest Service Timber and Watershed Laboratory Parsons WV 26287 USAd Smithsonian Environmental Research Center PO Box 28 Edgewater MD 21037 USAe Virginia Institute of Marine Science Rt 1208 Greate Road PO Box 1346 Gloucester Point VA 23062 USAf University of Maryland Center for Environmental Science Horn Point Laboratory PO Box 775 Cambridge MD 21613 USAg Department of Ecology and Evolutionary Biology Cornell University E311 Corson Hall Ithaca NY 14853 USAh Department of Ocean Earth amp Atmospheric Sciences Old Dominion University 4600 Elkhorn Avenue Norfolk VA 23529-0276 USAi Department of Anthropology 1111 Woods Hall University of Maryland College Park MD 20742-7415 USAj University of Maryland Center for Environmental Science Chesapeake Biological Laboratory 1 William St Solomons MD 20688 USAk Chesapeake Research Consortium 645 Contees Wharf Road PO Box 28 Edgewater MD 21037 USAl Penn State Cooperative Wetlands Center 216 Walker Building University Park PA 16802 USAm Cooperative Oxford Laboratory NOAA-NCCOS 904 South Morris Street Oxford MD 21654-1323 USA

a r t i c l e i n f o

Article historyReceived 3 March 2009Accepted 24 September 2009Available online 14 October 2009

Regional index termsUSAMid-Atlantic RegionThe Chesapeake Bay

Keywordsclimateestuariescirculationbiogeochemistryvascular plantsfisheries

Corresponding authorE-mail addresses najjarmeteopsuedu (RG Na

Pyke) mbadamsfsfedus (MB Adams) breitburgdsweethallwetlanvimsedu (C Hershner) kemphplucornelledu (R Howarth) mmulholloduedu (MR Mumdedu (M Paolisso) secorcblumcesedu (D Secordhw110psuedu (D Wardrop) BobWoodnoaagov

0272-7714$ ndash see front matter 2009 Elsevier Ltddoi101016jecss200909026

a b s t r a c t

We review current understanding of the potential impact of climate change on the Chesapeake BayScenarios for CO2 emissions indicate that by the end of the 21st century the Bay region will experiencesignificant changes in climate forcings with respect to historical conditions including increases in CO2

concentrations sea level and water temperature of 50ndash160 07ndash16 m and 2ndash6 C respectively Alsolikely are increases in precipitation amount (very likely in the winter and spring) precipitation intensityintensity of tropical and extratropical cyclones (though their frequency may decrease) and sea-levelvariability The greatest uncertainty is associated with changes in annual streamflow though it is likely thatwinter and spring flows will increase Climate change alone will cause the Bay to function very differentlyin the future Likely changes include (1) an increase in coastal flooding and submergence of estuarinewetlands (2) an increase in salinity variability on many time scales (3) an increase in harmful algae (4) anincrease in hypoxia (5) a reduction of eelgrass the dominant submerged aquatic vegetation in the Bay and(6) altered interactions among trophic levels with subtropical fish and shellfish species ultimately beingfavored in the Bay The magnitude of these changes is sensitive to the CO2 emission trajectory so thatactions taken now to reduce CO2 emissions will reduce climate impacts on the Bay Research needs includeimproved precipitation and streamflow projections for the Bay watershed and whole-system monitoringmodeling and process studies that can capture the likely non-linear responses of the Chesapeake Baysystem to climate variability climate change and their interaction with other anthropogenic stressors

2009 Elsevier Ltd All rights reserved

jjar) cpykeusgbcorg (CRsiedu (D Breitburg) carlmcesedu (M Kemp) rwh2ulholland) mpaolissoanth

) sellnerksiedu (K Sellner)(R Wood)

All rights reserved

1 Introduction

Estuarine ecosystems are vulnerable to human activities thatlead to nutrient pollution (and consequent eutrophication) excessor insufficient sedimentation dredging river water diversion andother types of pollution (UNEPGPA 2006) Estuaries are alsoparticularly vulnerable to climate change because they can respondto at least three different types of forcing (1) streamflow qualityand quantity (2) air-water fluxes of CO2 heat freshwater (ieevaporation and precipitation) and momentum (ie wind stress)and (3) fluctuations in sea level and other ocean properties With

76degW80degW

46degN

42degN

38degN

Chesapeake Bay watershed

Chesapeake Bay

VA

WV

PA

NY

MDDE

NJ

Atlantic O

cean

0 250 500125km

Fig 1 Map of the Chesapeake Bay its watershed and the states in its watershed

RG Najjar et al Estuarine Coastal and Shelf Science 86 (2010) 1ndash202

climate change due to human activity well underway (IPCC 2007)there is a need to assess the impacts of climate change on estuaries

The goal of this paper is to review and synthesize the scientificliterature on climate change impacts on the Chesapeake Bay (Fig 1)one of the largest and most productive estuaries in the world(National Oceanic and Atmospheric Administration 1990) In 2000commercial fisheries landings for the Chesapeake Bay exceeded US$172 million accounting for 5 of the value of all United Statesfisheries (National Marine Fisheries Service 2001) Although thesefigures are significant they understate the value of the ChesapeakeBay and its fisheries because they do not account for the ecologicaland recreational services the Bay provides to the food web andfisheries of the North American Atlantic Coast

In this review we follow a logical progression from changes inclimatic and hydrological forcing factors (Section 2) to changes inwatershed fluxes of nutrients and sediment (Section 3) and physical(Section 4) and biogeochemical (Section 5) conditions in the BayImpacts on Bay living resourcesdvascular plants (Section 6) and fishand shellfish (Section 7)dare then considered before summarizingour findings and providing general recommendations for furtherstudy (Section 8) Our synthesis builds on a number of reviews dis-cussing the impact of climate change on ecosystems coastal areasand marine resources of the mid-Atlantic region (Boesch 2008Moore et al 1997 Moss et al 2002 Najjar et al 2000 Rogers andMcCarty 2000 Wood et al 2002) the United States (Field et al2001 Scavia et al 2002) and the World (Kennedy et al 2002)

Statistically rigorous forecast probabilities are not possible atthis time for climate change and its impacts in the Chesapeake Bayregion By using expert judgment however we believe that we candefensibly assign rough probabilities regarding the direction ofmany changes To maintain consistency throughout this paper weuse the following terminology (Manning 2006) to express proba-bility ranges virtually certain (gt99) very likely (90ndash99) andlikely (66ndash90)

2 Climatic and hydrologic processes affecting the bay

21 Atmospheric composition

Being a well-mixed gas in the atmosphere regional and globalprojections of atmospheric CO2 are essentially identical Projectionsfor global mean atmospheric CO2 concentration over the next 100years vary widely mainly because of the uncertainty in future CO2

emissions (Fig 2) but also because of poorly understood feedbacksbetween climate and the carbon cycle However it is virtuallycertain that CO2 levels will continue to increase throughout the 21st

century Surface water CO2 changes are expected to closely trackatmospheric CO2 changes leading to a decrease in pH andcarbonate ion concentration [CO3

2] which is expected to harmCaCO3-secreting organisms including shellfish Orr et al (2005)showed that [CO3

2] and pH decreases averaging about 10 and 01respectively have already taken place throughout the surface oceandue to the invasion of anthropogenic CO2 Under a greenhouse gasscenario similar to the Intergovernmental Panel on ClimateChangersquos (IPCCrsquos) A2 storyline (Fig 2) these changes increase to45 and 05 respectively by 2100

22 Water temperature

Fig 3 shows 20th-century surface water temperature variabilitymeasured at two locations in the Chesapeake Bay High variability issuperimposed on a long-term warming the 1990s were about 1 Cwarmer than the 1960s Fig 3 also shows an estimate of surfacewater temperature averaged over the mainstem Bay based on datafrom the Chesapeake Bay Water Quality Monitoring Program

which sampled the water column at least monthly at several dozenstations throughout the mainstem Bay since 1984 The correspon-dence between the pier data and the Bay-average data during theperiod of overlap indicates that the longer time series measured atthe piers reflect mean Bay temperature quite well

Numerous studies have documented a positive correlationbetween water temperature in the Bay and regional atmosphericand oceanic temperature at time scales ranging from monthly todecadal (eg Cronin et al 2003 Preston 2004) It is therefore verylikely that the regional temperature projections made from climatemodels can be accurately applied to the Bay

Two recent studies have analyzed the output of global climatemodels (GCMs) in the Chesapeake Bay region (Hayhoe et al 2007Najjar et al 2009) Both studies found the multi-model average tocapture the 20th-century warming trend of the northern portion ofthe Chesapeake Bay watershed but the weak cooling observed inits southern portion (eg Allard and Keim 2007) is not found inmodels (Najjar et al 2009) The degree of projected warmingdiffers greatly among models (Fig 4) and scenario (Fig 5) but it isnevertheless very likely that temperature will continue to increasethroughout the 21st century Model-averaged projections in Najjaret al (2009) for the six scenarios shown in Fig 2 range from 3 to6 C warming by 2070ndash2099 (Fig 5a) When the best-performingmodels are used the projected warming decreases to 2ndash5 C(Fig 5c) Similar projections were found by Hayhoe et al (2007)

Changes in temperature extremes can be as important as annualmean temperature changes Meehl et al (2007) analyzed theoutput of nine global climate models for changes in heat wavesdefined as lsquolsquothe longest period in the year of at least five consecutivedays with maximum temperature at least 5 C higher than theclimatology of the same calendar dayrsquorsquo Under the A1B scenario(Fig 2) heat waves along the east coast of North America includingthe Mid-Atlantic are projected to increase by more than twostandard deviations by the end of the 21st century

0102030

Emis

sion

s(P

g C

yrminus1

)500

1000C

O2

(ppm

)

0

5

tem

pera

ture

cha

nge

(deg C

)

2000 2020 2040 2060 2080 21000

500

1000

Year

Sea

leve

lch

ange

(mm

)

A1B A1T A1FI A2 B1 B2

a

b

c

d

Fig 2 Global projections from the IPCC Third Assessment Report (TAR) except for panel (d) Six scenarios are shown described in Nakicenovic and Swart (2000) (a) CO2 emissions(b) modeled levels of carbon dioxide according to the Bern carbon cycle model (c) global mean surface air temperature change with respect to 1990 from the average of nine TARmodels Data for these figures was taken from Appendix II of Houghton et al (2001) (d) Global mean sea-level change with respect to 1990 using dHdtfrac14 a(T To) whereafrac14 34 mm yr1 C1 H is sea level T is the global mean air temperature and To is the temperature that is 05 C below the 1951ndash1980 average temperature (Rahmstorf 2007)Temperature change in panel (c) is with respect to 1990 1990 temperature was about 03 C greater than the 1951ndash1980 average We thus added 08 C to temperature in panel (c) toget T To This gives dHdt for 1990 of 17 mm yr1 which is within the error of the observed rate (Church et al 2004)

RG Najjar et al Estuarine Coastal and Shelf Science 86 (2010) 1ndash20 3

23 Precipitation

Climate models have similar Bay-region precipitation predictionsunder enhanced greenhouse gas levels (1) multi-model averages ofmore annual precipitation (Fig 5c and d) (2) a wide spread amongmodels of annual precipitation change (Fig 4) and (3) highconsensus among the models in winter and spring when precipi-tation is projected to increase (Fig 4) The wide spread in modeled

1930 1940 1950 1960 1970 1980 1990 2000 201013

14

15

16

17

18

year

Tem

pera

ture

(deg C

)

VIMS pierCBL pierBay average

Fig 3 Annual average surface temperature from the mouth of the York River (VIMSpier) the mouth of the Patuxent River (CBL pier) and average throughout the main-stem Bay (Bay average) VIMS pier data were reported in Austin (2002) and CBL pierdata were reported in Kaushal et al (in press) VIMS pier data are part of the VIMSScientific Data Archive and were acquired from Gary Anderson Virginia Institute ofMarine Science School of Marine Science College of William amp Mary Gloucester PointVirginia Bay average temperature was computed by David Jasinski Chesapeake BayProgram Office using surface temperature measurements from the Chesapeake BayWater Quality Monitoring Program Data were first average by month at each stationthen by year before taking arithmetic mean of all stations

annual precipitation changes reflects the Mid-Atlantic regionrsquosposition at the boundary between subtropical precipitationdecreases and subpolar precipitation increases consensus increasesfor the winter results as this boundary moves south (Meehl et al2007) The difficulty that climate model have had in capturing long-term trends in precipitation the Northeast US (Hayhoe et al 2007Najjar et al 2009)dmainly increasing and particularly in extremewet events (Groisman et al 2001 2004)dmay also be due to itslocation

An important characteristic of precipitation is its intensityparticularly for watershed export of sediment and phosphorus(Section 31) Defined as the annual mean precipitation divided bythe number of days with rain precipitation intensity in the Mid-Atlantic region is expected to increase under the A1B scenario(Fig 2) by one standard deviation by the end of the 21st century(Meehl et al 2007) This increase was found to be a result of anincrease in annual precipitation as well as the number of dry daysa finding consistent with changes in storm frequency and intensity(Section 26)

24 Streamflow

Much of the interannual variability in streamflow to the Bay isdriven by precipitation with a relatively small role for evapo-transpiration (eg Najjar 1999) The Northeast US including theChesapeake Bay watershed has been characterized as a region ofincreasing streamflow particularly in extreme wet events consis-tent with the precipitation trends (Groisman et al 2001 2004)

Previous hydrological modeling studies found widely varyingstreamflow projections in the Northeast USdfrom40 tothorn30 forroughly a doubling of atmospheric CO2 (summarized in Najjar et al2009)deven when forced by the same climate models (Neff et al2000 Wolock and McCabe 1999) The discrepancy in future projec-tions is most likely due to different modeled evapotranspiration

0

5

10

Tem

pera

ture

chan

ge (

deg C)

Winter Spring Summer Fall Annual-40

-20

0

20

40

Prec

ipita

tion

chan

ge (

)

CCCMCCSRCSIRECHMGFDLHADCNCARAverage

Fig 4 Annual and seasonal temperature (top) and precipitation (bottom) changes averaged over the Chesapeake Bay Watershed with respect to 1971ndash2000 predicted under the A2scenario for the period 2070ndash2099 (after Najjar et al 2009)


responses (and therefore streamflow responses) to temperaturechange This divergence is probably due to the lack of an observationalrecord of substantial temperature change with which to constrainhydrological models For example the standard deviation of annualair temperature over the Chesapeake Bay Watershed is 05 C (Najjaret al 2009) which is small compared with the multi-model meanprojected 100-year warming (Fig 5) Other confounding influences onstreamflow which are generally not considered in future projectionsinclude vegetation changes the direct influence of CO2 on evapo-transpiration and land use and land cover change

The seasonality of streamflow to the Chesapeake Bay isextremely important because it helps to regulate the timing of thespring phytoplankton bloom and is also very important in regu-lating nutrient delivery to the Bay (Section 51) Hydrological model

0

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nge

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)

Average of all models

2010minus39 2040minus69 2070minus990

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Time period

Prec

ipita

tion

chan

ge (

)

A1BA1TA1FIA2B1B2

a

c

Fig 5 Projected change in annual mean temperature (a and c) and precipitation (b and d)climate models (a and b) and the four ranked highest (c and d) (reproduced from Najjar et

simulations by Hayhoe et al (2007) in the US Northeast predictgreater wintertime flows (due both to snow melt and more rainrather than snow) and depressed summer flows (due to increasedevapotranspiration and also less groundwater recharge during thespring snowmelt period) They also predict an advance of the springstreamflow peak of nearly 2 weeks January-May average flow ofthe Susquehanna River is a significant predictor of summertimecirculation and biogeochemistry (Hagy 2002 Hagy et al 2004)Historically there is a strong correlation between January-May flowand precipitation in the Susquehanna River Basin such that frac-tional flow increases are equal to fractional precipitation increases(Najjar 2009) Given the consensus among models regarding springand winter precipitation increases it is likely that January-Mayflow of the Susquehanna River will increase in the future

Average ofCCCM ECHM HADC amp NCAR

2010minus39 2040minus69 2070minus99Time period

b

d

of the Chesapeake Bay Watershed for six IPCC scenarios (Fig 2) averaged over sevenal 2009)


Due to projections of a greater number of precipitation-free daysand greater evapotranspiration (resulting from higher tempera-tures) in the Bay watershed drought is expected to increase in thefuture Defining drought as a 10-or-more deficit of monthly soilmoisture relative to the climatological mean Hayhoe et al (2007)simulated increases in droughts of different durations over theNortheast US For example the number of short-term (1ndash3months) droughts was projected to increase 24ndash79 (based on theB1 amp A1FI greenhouse gas scenarios see Fig 2) by 2070ndash2099 withrespect to 1961ndash1990 Medium (3ndash6 months) and long (gt6 months)droughts had even larger fractional increases

25 Sea level

Tide gauge measurements reveal a steady increase in sea levelthroughout the Chesapeake Bay during the 20th century (Fig 6)Global-mean sea surface height increased at a rate of18 03 mm yr1 over the second half of the 20th century (Churchet al 2004) substantially smaller than Chesapeake Bay rateswhich range from 27 to 45 mm yr1 (average 35 mm yr1 nfrac14 6)(Zervas 2001) a difference most likely due to long-term subsi-dence (Davis and Mitrovica 1996) Rahmstorf (2007) developeda semi-empirical approach that predicts global sea-level increasesof 700 to 1000 mm by 2100 for a range of scenarios spanning B1 toA1FI (Fig 2d) Allowing for errors in the climate projections and inthe semi-empirical model the projected range increases to 500 to1400 mm If we add a Chesapeake Bay local component of2 mm yr1 to this we obtain sea-level increases of approximately700 to 1600 mm by 2100 a projection we consider to be very likely

Sea-level variability is very likely to increase in the future Asnoted below (Section 41) the tidal range is likely to increase asa result of increases in mean sea level in the Bay Further increasesin extreme wave heights will accompany the likely increases inintense storms both tropical and extratropical discussed next

26 Storms

Tropical cyclones and extratropical winter cyclones have haddramatic and long-lasting effects on the Chesapeake Bay For example50 of all the sediment deposited in the Northern Chesapeake Baybetween 1900 and the mid-1970s was due to Tropical Storm Agnes(June 1972) and the extratropical cyclone associated with the GreatFlood of (March) 1936 (Hirschberg and Schubel 1979)

1900 1920 1940 1960 1980 2000

minus02

minus01

0

01

02

year

Sea

leve

l ano

mal

y (m

)

BaltimoreSewells Point

Fig 6 Long-term sea-level change at two locations in the Chesapeake Bay BaltimoreMD (upper bay) and Sewells Point VA (lower bay) Data are annual mean differencesfrom the 1950ndash2000 average and were acquired from NOAArsquos Center for OperationalOceanographic Products and Services

Trenberth et al (2007) summarized recent studies on tropicalcyclone trends noting a significant upward trend globally in theirdestructiveness since the 1970s which is correlated with seasurface temperature Christensen et al (2007) and Meehl et al(2007) summarized future projections in tropical cyclonesconcluding that it is likely that peak wind intensities will increase

Past and future trends in extratropical cyclones are fairly clear atthe hemispheric scale but not at the regional scale There is goodevidence that mid-latitude winter storm frequency decreased andintensity increased over the second half of the 20th century (egPaciorek et al 2002) However an analysis of US East Coastextratropical winter storms showed no significant trend infrequency and a marginally significant decline in intensity (Hirschet al 2001) Lambert and Fyfe (2006) showed remarkable consis-tency among GCMs in the future projections of winter extratropicalcyclone activity For the A1B scenario (see Fig 2) the multi-modelmeans over the Northern Hemisphere are a 7 decrease infrequency of all extratropical winter cyclones and 19 increase inintense extratropical winter cyclones comparing the 2081ndash2100period to the 1961ndash2000 period In a study focused on NorthAmerica Teng et al (2007) tentatively suggested that cyclonefrequency in the Northeast US will decrease

3 Fluxes of nutrients and sediment from the watershed

The fluxes of sediments and nutrients from the landscape havebeen profoundly affected by climate variability and so it isreasonable to expect that future climate change will alter materialfluxes to the Bay Most of the nutrient inputs to the Chesapeake Baycome from non-point sources such as agriculture and atmosphericdeposition In this section we address non-point source (NPS)sediment and nutrient pollution with particular emphasis onatmospheric deposition because of the large uncertainties involvedWe also consider how climate change may influence the roles ofwetlands and point sources in nutrient loading to the Bay

31 Non-point pollution by sediments and phosphorus

Because most NPS phosphorus pollution is particle bound thecontrols on sources and fluxes of sediments and phosphorus aresimilar (eg Sharpley et al 1995) The major control on NPS sedi-ment and phosphorus pollution is the rate of erosion which isinfluenced by the interaction of land-use patterns and climate (egMeade 1988) Erosion rates from forest ecosystems are quite lowwhereas erosion from agricultural and developed lands can be veryhigh (Swaney et al 1996) Erosion occurs when water flows overthese surfaces and that in turn occurs when soils are saturatedwith water or during major precipitation or snowmelt events

Annual sediment loading to the Chesapeake Bay is a non-linearfunction of annual streamflow (Fig 7) indicating an increase intotal suspended sediment concentration as flow increases whichlikely results from enhanced erosion and resuspension of sedi-ments in the streambed Even if the mean discharge were to remainunchanged erosion could increase if the precipitation intensitywere to increase a projection that is more certain than annualstreamflow changes (Sections 23 and 24) To date there has beenlittle if any testing of how various climate change scenarios mayaffect erosion in the watersheds of the Chesapeake Bay

NPS phosphorus pollution is a function of the amount of phos-phorus associated with eroded soils in addition to the rate oferosion Agricultural soils are elevated in phosphorus compared toforest soils because of the addition of inorganic fertilizers andmanure (eg Sharpley et al 1995) Not only can erosion of these P-rich agricultural soils be a major source of phosphorus pollutionbut the problem remains as agricultural lands are converted into

0 500 1000 1500 2000 2500 30000

100

200

300

400

flow (m3 sminus1)

sedi

men

t flu

x (k

g sminus1

)

Fig 7 Relationship between annual sediment yield and total freshwater inflow to theChesapeake Bay for the years 1990 to 2004 The curve is a least-squares exponential fit(r2frac14 093) yfrac14 513e00015x The estimates were obtained from the Chesapeake BayProgram web site The annual sediment yields were computed by the United StatesGeological Survey by summing the products of daily streamflow and riverine TotalSuspended Solids (TSS) concentrations The TSS concentrations are based on a statis-tical model calibrated with TSS observations from several monitoring stations Detailson the data sources and methodology are given in Langland et al (2006 pg 13)

0 2 4 6 8 10 12 140

0 2

0 4

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Temperature (deg C)

0 200 400 600 800 1000 1200 14000

0 2

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Precipitation (mm yr-1)

0 200 400 600 8000

0 2

0 4

0 6

Discharge (mm yr -1)

Frac

tiona

l Del

iver

y of

NAN

I

Fig 8 The fractional delivery of net anthropogenic nitrogen inputs (NANI) for 16major watersheds in the northeastern United States plotted as a function of meandischarge mean precipitation and mean temperature The relationship for dischargeand precipitation are highly significant (pfrac14 0003 and 00015 respectively) the rela-tionship for temperature is weaker (pfrac14 011) (reprinted from Howarth et al 2006)


suburban landscapes phosphorus losses can be particularly greatfrom erosion at construction sites when the soils are former agri-cultural soils and even storm water retention ponds and wetlandscan be major sources of NPS phosphorus pollution if the systemsare constructed with P-rich soils (Davis 2007)

32 Non-point pollution by nitrogen

Nitrogen NPS pollution is controlled by an interaction of nitrogeninputs to the landscape and climate For many large watersheds inthe temperate zone including the major tributary rivers of theChesapeake Bay the average export flux of nitrogen from a water-shed is 20ndash25 of the net anthropogenic nitrogen inputs (NANI) tothe watershed where NANI is defined as the use of syntheticnitrogen fertilizer nitrogen fixation associated with agro-ecosys-tems atmospheric deposition of oxidized forms of nitrogen (NOy)and the net input of nitrogen in foods and feeds for humans and foranimal agriculture (eg Boyer and Howarth 2008) However thepercentage of NANI that is exported out of a watershed in rivers isrelated to climate For example Boynton and Kemp (2000) showedthat years with high runoff resulted in enhanced nutrient exportfrom the Chesapeake Watershed Castro et al (2003) modelednitrogen fluxes to the major estuaries of the United States includingthe Chesapeake Bay as a function of NANI land use and climateTheir models suggested that land use is a very important factor indetermining export of NANI with greater export from urban andsuburban landscapes and much lower export from forests and thatland use and climate may interact strongly

Howarth et al (2006) compared the average percentage export ofNANI across 16 major river basins in the northeastern US (Fig 8) Inthe watersheds where precipitation and river discharge is greaterthe percentage of NANI that flows downriver to coastal ecosystems isup to 40ndash45 while in drier regions only 10ndash20 is exported overlong-term periods Howarth et al (2006) attributed this to sinks ofnitrogen in the landscape with less denitrification in the wetterwatersheds due to lower water residence times in wetlands and low-order (ie headwater) streams Given the climate change predictionsfor increased precipitation presented by Najjar et al (2000) andassuming no change in NANI or land use Howarth et al (2006)predicted an increase in nitrogen flux down the Susquehanna Riverof 17 by 2030 and 65 by 2095 (associated with precipitation

increases of 4 and 15 respectively) More updated precipitationprojections for the Susquehanna River Basin (eg Fig 4 Hayhoeet al 2007 Najjar et al 2009) would yield similar results

Schaefer and Alber (2007) expanded on the analysis of Howarthet al (2006) by including data from the major watersheds in thesoutheastern US This larger data set showed a significant influenceof temperature with low percentage export of NANI at hightemperatures and a greater percentage export of NANI at lowtemperatures Schaefer and Alber (2007) attributed the temperatureeffect on export to act through denitrification with warmth favoringhigher rates The significant correlation with temperature observedby Schaefer and Alber (2007) is driven by the large temperaturedifference between the northeastern and southeastern regionsOther controlling factors such as soil types may be at play across thislarger data set If temperature is the major factor controlling the

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Nitrogen loading (103 kg yr minus1)

Annual precipitation (cm)

Annu

al m

ean

air t

empe

ratu

re (

deg C)

90 100 110 120

13

14

15

16

17

18

Fig 9 Simulated annual nitrogen loading in the Western Branch of the Patuxent RiverMaryland as a function of annual precipitation and annual mean temperature Theblack circle represents recent average conditions Model output is from Johnson andKittle (2007)


percentage export of NANI as Schaefer and Alber (2007) concludea warming of 3 C would decrease the nitrogen flux down the Sus-quehanna by about 20 a trend opposite that predicted by Howarthet al (2006)

Process-based simulation models of biogeochemical cycling inwatersheds offer another approach for assessing the impact ofclimate change on riverine N export to coastal waters However thecurrent level of uncertainty about the importance of underlyingmechanisms that relate nitrogen flux to climatic controls inherentlylimits the usefulness of such models Another weakness of process-based models is that they treat organic forms of nitrogen poorlyThough much of the flux of nitrogen in rivers occurs as inorganicnitrogen increased atmospheric deposition can increase the exportof organic nitrogen from forests (Brookshire et al 2007)

The only process-model-based climate change study of N exportwe are aware of in the watersheds of the Chesapeake Bay is that ofJohnson and Kittle (2007) who simulated the response of annualnitrogen loading in the Western Branch of the Patuxent RiverMaryland to changes in annual mean air temperature and precipi-tation (Fig 9) They find that N export decreases by about 3 fora temperature increase of 1 C and increases by 5 for a precipita-tion increase of 5 (Fig 9) sensitivities that are much smaller thanthose found by Howarth et al (2006) and Schaefer and Alber (2007)

33 Atmospheric deposition of nitrogen

Atmospheric N input to the Bay occurs both directly onto thesurface of the Bay and onto the landscape with subsequent exportto the Bay and includes both wet deposition and deposition as drygases and particles Unfortunately the dry deposition of manyabundant nitrogen pollutant gases (such as NO NO2 HONO andNH3) is not measured in any of the national deposition monitoringprograms (NADP CASTnet or AIRMon) Total N deposition istherefore poorly known with estimates ranging from 14 to 64 ofthe total nitrogen load to the Chesapeake Bay (Castro et al 2003Howarth 2006)

Climate change may alter both the pattern of nitrogen deposition(due to changes in reaction kinetics precipitation and windpatterns) and the retention of nitrogen once it is deposited (Section32) Climate change could also influence this partitioning throughimpacts on growth and productivity of forests which have been

shown to strongly influence the retention of deposited nitrogen(Aber et al 1998) The impact of these changes may be mediated byforest disturbances such as gypsy moth outbreaks (Eshleman et al2000) which may also be sensitive to climatic variation and change(Gray 2004)

Climate change also will eventually lead to major changes in thespecies composition of forests and these changes are also likely toinfluence nutrient export Modeling studies have suggested thathabitat for some tree species within the Chesapeake Bay watershedwill increase such as red maple sweetgum and loblolly pine whileother currently plentiful species are expected to decline such asblack cherry American beech and other oaks (Iverson et al 2005)Tree species composition and the resulting litter quality areimportant factors in controlling variation in N cycling in temperateforest soils (eg Lovett et al 2002) Greater abundances of sugarmaple and striped maple for example were associated withgreater net nitrate production in soils relative to coniferous trees(Venterea et al 2003) making stands dominated by maple speciesmore susceptible to losses of nitrate to surface waters

34 Freshwater wetlands

Freshwater wetlands are the interface between human activitiesin uplands and the streams and rivers of the Chesapeake Baywatershed and thus have the potential to regulate nutrient inputsto the Bay Of particular importance for nutrient abatement in theChesapeake Bay Watershed are riparian ecosystems along low-order streams (Lowrance et al 1997) Most wetland processes aredependent on catchment-level hydrology (Gitay et al 2001) andare therefore susceptible to climate change Potential impacts rangefrom extirpation to enhancement and include alterations incommunity structure and changes in ecological function (Burkettand Kusler 2000) There is evidence that wetlands that dependprimarily on precipitation for their water supply may be morevulnerable to climate change than those that depend on regionalgroundwater systems (Winter 2000) The number and complexityof factors that influence wetland occurrence and type make itdifficult to predict the fate of wetlands directly from knowledge oftemperature and precipitation changes alone Needed are predic-tions of change in hydrology that will be induced by both climateand land cover change For example the hydrologic impacts fromchanges in rainfall patterns will depend on the amount and locationof impervious surfaces in the watershed

Climate-induced impacts to wetlands will be layered onto analready compromised state of the resource An assessment ofwetland condition in the Upper Juniata River Watershed PA (War-drop et al 2007b) reported that over 68 of the total wetland areawas in medium or low condition correlated with increasing agri-culture and urbanization in the watershed Two regional assess-ments of wetland condition indicate that the ability of wetlands inboth the Upper Juniata and Nanticoke River watersheds to performvaluable functions such as removal of inorganic nitrogen andretention of inorganic particulates is already significantly reduced(Wardrop et al 2007a) with the majority of wetlands functioningbelow reference standard levels These impacts are expressedprimarily by modification of supporting hydrology (Brooks et al2004) Climate-induced hydrologic regime changes may simplystress these systems further resulting in their decreased capacity toserve important ecotone functions

35 Point source pollution

Growing human populations are very likely to interact withchanges in climate to alter discharge from point sources of pollutionsuch as water treatment plants industrial facilities and urban storm


water systems Theoretical relationships and studies from otherregions suggest the potential for significant impacts but little workhas been done in the Chesapeake Bay Watershed A screeningassessment of the potential impact of climate change on combinedsewer overflow (CSO) in the Great Lakes and New England found thatCSO systems are designed to function under historical precipitationregimes (US Environmental Protection Agency 2008b) Designcapacity of CSO systems was found to be linearly proportional toanticipated precipitation intensity as a consequence significantincreases in precipitation intensity (Section 23) will likely under-mine design assumptions and increase the frequency of overflowevents A similar analysis for publicly-owned treatment works(POTWs) found that the operations of POTWs are sensitive to boththe volume of incoming effluent and the hydrologic condition ofreceiving waters (US Environmental Protection Agency 2008c)Climate change could therefore have a significant effect on bothNational Pollutant Discharge Elimination System (NPDES) permit-ting and POTW financing

4 Bay physical response

41 Circulation

There are no direct measurements of estuarine circulation in theChesapeake Bay that document the influence of climate variabilityRather measurements of temperature and salinity have been usedto quantify stratification and infer circulation patterns and rates ofmixing Hagy (2002) analyzed mainstem Bay salinity and temper-ature data to show that the AprilndashSeptember average stratificationin the mid-Bay is strongly and positively correlated to the JanuaryndashMay average Susquehanna River flow Given the likely increase inthis flow in the future (Section 24) it appears that summer-timestratification increases are likely as well It is unlikely that warmingwill enhance this stratification significantly because the time scaleof climate change is expected to be long enough that the Bay asa whole will warm Hagyrsquos (2002) diagnostic box modeling of thecirculation also showed that the summer-averaged landwardadvection below the pycnocline into the middle Bay increases withthe JanuaryndashMay average Susquehanna River flow

The only numerical modeling study to consider the impact ofclimate change on Chesapeake Bay circulation is the recent study byZhong et al (2008) This study suggests that the tidal range nearBaltimore Maryland which is in the upper portion of the Ches-apeake Bay will increase by 15ndash20 if sea level increases by 1 mZhong et al (2008) argued that this amplitude increase is caused byfriction reduction and the Bay being closer to its resonant periodIncreases in tidal range are very likely to be accompanied byincreases in mixing and shoreline inundation

42 Salinity

Salinity variations throughout the Bay have been shown to bestrongly tied to streamflow (eg Schubel and Pritchard 1986)Gibson and Najjar (2000) estimated that a change in annual Sus-quehanna River flow of 10 would result in a change in annual meansalinity (of opposite sign) of about 1 4 and 7 in the lower middleand upper mainstem Bay respectively Maximum change in saltconcentration is in the central Bay approximately 06 for a 10flow increase With projected annual flow changes by the end of the21st century of 40 to thorn30 (Section 24) annual mean salinity inthe central Bay could change by as much as 2 in either direction

Salinity variability is very likely to be affected by climate changeGiven the projected increases in JanuaryndashMay flow of the Susque-hanna River (Section 24) we can expect a decrease in mean salinityduring the winter and spring summer and fall projections are

much more uncertain Saltwater intrusion events with durationsgreater than 1 month are likely to increase because of the projectedincreases in drought frequency (Section 24)

Only one study has attempted to quantify salinity variationsdue to sea-level rise in the Chesapeake Bay After accounting forstreamflow variations Hilton et al (2008) found significant trendsin about half of the volume of the mainstem Chesapeake Baybetween 1949 and 2006 during which average sea level in the Bayrose by about 02 m The mean salinity change in these regions wasabout 08 at least half of which could be explained by sea-levelrise according to hydrodynamic model simulations Givena salinity sensitivity to sea level of about 04 O 02 mfrac14 2 m1a sea-level rise of 07 to 16 m by 2100 (Section 25) would increasesalinity by 14 to 32

43 Suspended sediment

Excess sediment contributes substantially to the Bayrsquos poorwater quality (Langland et al 2003) the majority of this sediment isnon-volatile (Cerco et al 2004) and this non-volatile component ismainly delivered by rivers (Smith et al 2003) In 2003 The Ches-apeake Bay Program (CBP) proposed to reduce land-based sedi-ment loading 18 by 2010 in order to achieve the water clarityneeded for underwater grasses to survive (Chesapeake BayProgram 2003) A least squares fit to the data in Fig 7 yieldsa sediment load of 87 kg s1 for the mean streamflow of 1890 m3 s1

during 1990ndash2004 Using projected flow changes by the end of the21st century of40 tothorn30 (Section 24) we estimate that the meansediment load could increase to 200 kg s1 more than a doubling ordecrease to 28 kg s1 less than a third of the current load Thusclimate change has the potential to either undo efforts to meetwater clarity goals or make it much easier to reach them As notedabove more intense precipitation in fewer events will probablyincrease sediment loading but the sensitivity is unknown

In addition to natural and anthropogenic processes in watershedsthat influence suspended sediment concentrations in rivers estua-rine suspended sediment is controlled by a variety of processes theamount of streamflow entering the estuary shoreline erosion in situbiological production and decomposition the re-suspension ofparticulate matter through currents (driven by winds tides andbuoyancy forces) the redistribution by advection and mixing withinthe estuary and the rate of sedimentation Many of these controls arealso sensitive to climate but quantitative relationships that linkclimate change to changes in sediment fluxes are lacking

5 Estuarine biogeochemistry

51 Nutrient cycling and plankton productivity

While dominated by diatoms throughout the year (Adolf et al2006) phytoplankton production and species composition in theChesapeake Bay generally follow predictable seasonal patternsdictated primarily by streamflow light and temperature (Maloneet al 1996 Marshall and Nesius 1996) Meteorology through riverdischarge governs spring bloom timing and extent (Harding 1994)During the relatively low-light cold and turbulent winterearly-spring period centric diatoms dominate the flora (Sellner 1987) Asnutrients delivered by the spring freshet are exhausted from thesurface waters a substantial fraction of the spring diatom bloomsinks (mostly as intact cells) through the pycnocline Thereaftersurface summer productivity is supported by nutrient supply frombelow the pycnocline via wind-induced destratification and pyc-nocline tilting (Malone 1992) During the warm stable summermonths the algal community shifts to a mixture of picoplanktonsmall centric diatoms and flagellates (Malone et al 1986)


Aperiodic dinoflagellate blooms are also frequent and some taxasuch as Pfiesteria spp and Karlodinium veneficum (Place et al 2008)may exert toxic or other harmful effects At this time primaryproductivity (most of which is due to rapid nutrient recycling)microzooplankton grazing zooplankton production and fishproduction (Section 73) are high

Climate change has the potential to alter Bay phytoplanktondynamics through changes in precipitation and the intensity andfrequency of storms The projected winterndashspring precipitationincreases for the Bay watershed (Fig 4 Section 23) will likelyincrease nutrient loading during these seasons leading to higherplanktonic production Higher temperatures will likely result in anearlier spring bloom which could cause trophic uncoupling orchange the spatial distributions of particular taxa (eg Edwardsand Richardson 2004) The summer period depicted as moredrought-likely (Section 24) could be typified by sporadic highintensity storms and discharge events If storms are overlandresulting discharge would lead to increases in short-term stratifi-cation and increased preponderance of algal blooms (Mulhollandet al 2009) including some of the problematic taxa identifiedabove Should the intense storms pass over the Bay and tributariesmixing of the water column would very likely occur probably withincreased discharge this would yield optimal conditions for diatomgrowth similar to the fall bloom in the mesohaline Bay (Sellner1987) Climate change might therefore result in an annual phyto-plankton cycle with a larger-than-average spring diatom bloomfollowed by small cells during the summer drought interspersedwith aperiodic dinoflagellate blooms or diatom maxima afterstorms pass Species diversity already appears to have increasedover the last 20 years (Marshall et al 2004) as has chlorophylla (Kemp et al 2005) effects largely attributed to eutrophicationThe continued interactive effects of climate change and eutrophi-cation on these already documented changes are likely to beprofound in the Chesapeake Bay system

52 CO2 and temperature effects on plankton

CO2 is the preferred form of carbon for the principle carbon-fixing enzyme ribulose-15-bisphosphate carboxylasendashoxygenase(Rubisco) however most of the dissolved inorganic carbon inseawater is bicarbonate ion (HCO3

) As a consequence most cellshave various carbon concentrating mechanisms (CCMs) in order toconcentrate CO2 near active Rubisco sites Species without CCMsare very likely to benefit directly from increases in CO2 (Section 21Fig 2b) Further there are different forms of Rubisco with differentaffinities for CO2 Some of the bloom-forming microalgae aredinoflagellates that appear to have a form of Rubisco that has a lowaffinity for CO2 compared with the Rubisco found in most othermicroalgae (eg Ratti et al 2007) CO2 increases might alleviatecarbon limitation of Rubisco and allow higher growth rates of thesedinoflagellates thereby increasing the number of harmful algalblooms throughout the system While high CO2 might result inincreased C productivity enhanced carbon fixation does not resultin enhanced nutrient drawdown or increases in the C content ofcells (Riebesell et al 2007) Rather enhanced carbon fixation atleast in some cases results in release of dissolved organic carbon(DOC) that could fuel microbial heterotrophs and change netsystem metabolism (Riebesell et al 2007)

CO2 has been shown to stimulate diatom growth relative to that ofprymnesiophytes in incubations of natural communities (Tortell et al2002) and it has stimulated dinoflagellate (Rost et al 2006) cocco-lithophorid (Riebesell et al 2007) and cyanobacterial growth incultures Synechococcus growth as well as growth of the raphidophyteHeterosigma akashiwo were stimulated under both high CO2 and hightemperature scenarios (Fu et al 2008ab) whereas the common late

spring bloom-former Prorocentrum minimum was less affected (Fuet al 2008b)

Temperature increases are very likely to affect the metabolicstatus of the Chesapeake Bay In a synthesis of microbial ratemeasurements in the Chesapeake Bay Lomas et al (2002) foundthat planktonic respiration increases with temperature morerapidly than photosynthesis Their results suggest that the Baymight become net heterotrophic on an annual time scale reversingits current net autotrophic status (Smith and Kemp 1995) Thecombined impact of excess DOC release by phytoplankton due tocarbon overproduction and increasing temperature are very likelyto have significant effects on the microbial community

53 Harmful algal blooms and pathogens

Peperzak (2003) conducted several experiments with brackishbloom-forming and non-bloom forming taxa under simulatedstratified conditions and a 4 C temperature increase Skeletonemacostatum a common winterndashspring taxon in the Bay was not per-turbed by the shift to stratified conditions However P minimumthe spring co-dominant in the Chesapeake (and occasional toxinproducer) and two raphidophytes (Heterosigma spp) found in mid-Atlantic coastal bays were stimulated by the increased stratifica-tion and temperature

Shifts in algal taxonomic composition from flow-inducedstratification pose potential problems both in terms of altered foodweb structure and toxicity to trophic groups Several taxa canreduce zooplankton grazing and fecundity due to poor food quality(eg Harvey et al 1989) or the production of toxins or grazing-deterrent compounds (eg Adolf et al 2007) Very high cellabundances can also reduce grazing pressure from co-occurringzooplankton populations (eg Sellner and Olson 1985) Pelagicbacterial production may increase as well due to surface-mixed-layer lysis of dinoflagellates (Sellner et al 1992) favoring hetero-trophic flagellates rather than copepods The net result is anincrease in the importance of the microbial food web rather thanthe classical food chain that supports fish production

Additional impacts of an altered climate specifically prolongeddroughts (Hayhoe et al 2007) will result in increased oceanicintrusion into the Bay introducing coastal populations of phyto-plankton including several harmful taxa This occurred during thespring and summer of 2002 following drought conditions in 1999ndash2002 coastal populations of Dinophysis acuminata were delivered tothe lower Potomac River estuary (Marshall et al 2004) resulting infears for okadaic acid intoxication and diarhettic shellfish poisoningfor the oyster-consuming public During summer 2007 prolongeddrought followed by storm events that resulted in overland run-off isthought to have triggered an extensive bloom of Cochlodinium pol-ykrikoides in the lower Chesapeake Bay (Mulholland et al 2009)Alexandrium monolitum common in the Gulf of Mexico but previ-ously undetected in the Chesapeake Bay watershed appeared toenter the Chesapeake Bay and co-occurred with C polykrikoidesduring the latter half of this bloom (Marshall and Egerton 2009)

The leafy chlorophyte Enteromorpha a macroalga is stimulatedby elevated water temperatures (Lotze and Worm 2002) and alongwith a similar taxon Ulva is characteristic of eutrophic estuariesincluding the Chesapeake and its tributaries It is conceivable thatwarmer winters and springs might favor earlier growth of these twomacroalgae and contribute to fouling of shorelines and submergedvegetation clogging of commercial fish nets and hypoxic conditionsin sheltered bays Further the decay of these blooms has beenassociated with the onset of other harmful algal blooms in otherareas such as Aureococcus anophagefferens (Kana et al 2004)

Harmful bacteria will also respond to temperature changes Sometrue heterotrophic bacteria like the Vibrio species are associated


with serious illnesses such as gangrene and sepsis Pathogenicspecies such as Vibrio vulnificus and Vibrio cholerae have beenidentified in Chesapeake Bay waters (summarized in Rose et al2000) Vibrio cholerae and Vibrio parahaemolyticus appear to beassociated with elevated sea surface temperatures (Colwell 1996McLaughlin et al 2005) Further growth of a free-living strain of thisbacterium has been shown to be stimulated by a coastal dinofla-gellate bloom off of California (Mourino-Perez et al 2003) reachinglevels three orders of magnitude higher than the known minimuminfectious dose Therefore climate-induced increases in harmfulalgal blooms may increase threats to human health either directly orby fueling pathogen growth with bloom-derived organic matter

Shellfish ingestion and concentration of pathogenic bacteria canalso lead to outbreaks of gastroenteritis and with V vulnificusdeath in some human consumers (Rose et al 2000 and referencestherein) Increasing temperatures in the Chesapeake Bay wouldfavor these bacteria (as noted above) increasing the threat of thisdisease in the basin

54 Dissolved oxygen

Dissolved oxygen levels have become a central indicator of theoverall health of the Chesapeake Bay Hypoxia is caused by thecombination of the sinking of the spring phytoplankton bloomwhich fuels bottom respiration and density stratification whichinhibits mixing that would otherwise replenish the deeper waterswith oxygen (Malone 1992) The significant trend of increasingintensity duration and extent of hypoxic conditions since 1950 isrelated to increased nutrient loading from human activities in thewatershed (Hagy et al 2004)

Climate influences the spatial and temporal distribution ofhypoxic conditions Hagy et al (2004) found JanuaryndashMay averageflow of the Susquehanna River to be a good predictor of thesubsequent summertime volumes of low-oxygen water Theirfunctional fits to the data suggest that a 10 flow increase willincrease the volume of anoxic water (lt02 mg l1) by 10 severelyhypoxic water (lt10 mg l1) by 6 and mildly hypoxic water(lt20 mg l1) by 3 Thus if JanuaryndashMay Susquehanna River flowincreases throughout the 21st century as expected (Section 24)then it is very likely that summertime oxygen levels will decline

Lower O2 solubility associated with warming would contribute tofurther reductions in bottom water O2 concentrations The sensitivityof the oxygen saturation concentration to temperature at thetemperature of sub-pycnocline waters in July (w20 C) is 016mg l1 C1 The difference in oxygen concentration betweenseverely hypoxic and anoxic waters as defined above is 08 mg l1Thus a warming of even 5 C could make waters that are currentlyseverely hypoxic turn anoxic solely due to solubility effects

Higher temperatures would also tend to accelerate rates ofnutrient recycling further stimulating phytoplankton productionand potentially deposition (eg Kemp et al 2005) This coupledwith the CO2-stimulated phytoplankton production and the sug-gested shift towards greater heterotrophy with warming (Section52) would tend to drive oxygen concentrations even lowerSimulation modeling studies for the northern Gulf of Mexicosupport these hypothesized responses of bottom water hypoxia toclimate change scenarios (Justic et al 2003)

6 Vascular plants

61 Submerged aquatic vegetation

Availability of light is a primary factor regulating submergedaquatic vegetation (SAV) abundance and spatial distribution in theChesapeake Bay (Kemp et al 2004 2005) A major decline in the

Bayrsquos SAV abundance which began in the mid-1960s (Orth andMoore 1983) was very likely due to widespread decreases in lightavailability that resulted from increases in suspended sediments andnutrient-stimulated growth of planktonic and epiphytic algaethroughout the estuary (eg Moore and Wetzel 2000) The lightsensitivity of SAV may be particularly problematic in the context ofclimate change given the possibility of significant increases in sedi-ment loading resulting from greater and more episodic precipitation(Sections 23 and 31)

SAV species exhibit widely varying sensitivity to temperaturewith optimal growth ranges of 22ndash25 C for eelgrass (Bintz et al2003) and 30ndash35 C for various freshwater plants growing inbrackish habitats (Santamarıa and van Vierssen 1997) It is antici-pated that higher temperatures will favor some species over others(Ehlers et al 2008) However a massive summer 2005 die-off inChesapeake Bay eelgrass which was triggered by an extended hotperiod with daily peak water temperatures exceeding 33ndash35 Cunderscores the fact that the Bay is near the southern geographiclimit for this species (Moore and Jarvis 2008 Orth et al review)With heat waves projected to increase in the Mid-Atlantic region(Section 22) we can expect such die-offs to become more frequentin the future It appears that high temperatures and low watercolumn mixing may contribute to internal oxygen deficiencydegradation of meristematic tissue and mortality for eelgrass andother SAV (Greve et al 2003) Higher temperatures may also favorgrowth of epiphytic algae to further reduce light available to SAV(Bintz et al 2003 Short and Neckles 1999)

Inter-annual variations in SAV distribution and abundance in theChoptank River estuary and other mesohaline Bay areas appear tocorrespond to fluctuations in freshwater flow In general decreasedriver flow results in reduced nutrient and sediment loading clearerestuarine water and higher plant growth (Stevenson et al1993) Forexample a resurgence of the euryhaline widgeon grass (Ruppiamaritima) occurred in the lower Choptank estuary after threesuccessive drought periods (2ndash4 years each) during the 1980s andearly 1990s (Kemp et al 2005 Orth et al review) This pioneer SAVspecies which is sensitive to variations in light availability andreproduces annually from an established seed bank has expanded inthis region to form generally stable monospecific stands (Orth et alreview) These R maritima beds however exhibit year-to-year fluc-tuations in abundance that correlate well with spring river flow andsummer water clarity (Fig 10) In previous decades this region wasalso populated by a more diverse community of SAV comprisedmostly of freshwater species that have varying tolerance for brackishconditions (eg Moore et al 2000) For many of these plant speciesincreased salinity associated with reductions in river flow canproduce osmotic stress that precludes their growth and survivaldespite improved water clarity (Stevenson et al1993) Thus climate-induced variations in freshwater flow would tend to affect SAVcommunities in middle regions of the Bay and its tributaries incomplex ways with abundance of salt-tolerant species expandingbut species diversity declining under reduced flow conditions

In addition to temperature salinity and light SAV is sensitive topH and CO2 concentration Although eelgrass was shown toincrease productivity under elevated CO2 levels with a 25increase in biomass under a doubling of CO2 there was no responseunder light-limiting conditions (Palacios and Zimmerman 2007)Earlier studies showed that responses vary substantially withduration of exposure to CO2 levels (eg Thom 1996) The morerecent results however suggest that CO2 increases could aid SAVrestoration efforts but only if measures are also taken to maintainsufficient water clarity In addition SAV species vary widely in theiraffinities for both carbon dioxide and bicarbonate (eg Beer andKoch 1996) suggesting that higher CO2 levels may contribute toalteration in species composition

0

1

2

)m( htpe

D ihcceS

15

05

25

3

1996 1998 2000 2002 2004

)ah( aerA VAS

0

200

1000

)m( htpe

D ihcceS

00

05

10

15

20

100 200 300 700Mar - May avg Discharge (m3 min-1)

r2 = 075

800

600

400

SAV

Secchi

600500400

Fig 10 Comparison of Choptank River flow and Secchi depth and areal cover forsubmersed aquatic vegetation (SAV primarily Ruppia maritima) in the lower Choptankestuary over a recent 9-year period SAV areal cover (aerial photos from single last summerflight generally July or August) and Secchi depth (Station EE21 April-October means) dataare from chesapeakebaynet and Choptank river flow data are from wwwwaterdatausgsgov Variation in SAV cover is significantly (plt 003) related to Secchi depth as followsSAVfrac14 633 Secchi ndash 203 (r2frac14 050) and variation in Secchi depth is significantly(plt 0002) related to river flow as follows Secchifrac14 195ndash0003 Flow (r2frac14 075)


62 Estuarine wetlands

Inundation by rising sea levels is one of the most direct threatsfaced by coastal and estuarine wetlands in the Chesapeake Bayregion The amount of land inundated by a given sea-level rise isa complex function of elevation shoreline geology land use landcover wetland ecology and the rate of sea-level rise Two studieshave estimated inundation for the Chesapeake Bay shoreline andboth have relied on elevation alone Titus and Richman (2001) usingDigital Elevation Models (DEMs) and shoreline data estimated thatabout 2500 km2 of land is below the 15-m elevation contour inVirginia and Maryland (essentially the shores of Chesapeake Bay)Wu et al (2009) used DEMs with finer (30-m) horizontal resolutionto estimate that 1700 km2 of land in Virginia and Maryland liesbelow the 07-m contour about half of which is wetlands

The current forecasts for the rates of sea-level rise in the Ches-apeake Bay are significantly greater than rates experienced for thelast several centuries (Section 25) At present it is not clear howmuch of the existing wetland complement in the Bay region will beable to either accrete vertically or migrate horizontally fast enoughto keep pace with the accelerated rate of change (eg Kearney et al1994) Extensive wetlands along the mainstem of the Bay such asthe Blackwater Wildlife Refuge in Maryland and the GuineaMarshes in Virginia are already showing decreased areas of vege-tative cover as a result of inundation and erosion Extensive oxbow

wetlands at the headwaters of the Bayrsquos tidal tributaries are alsoundergoing changes in vegetative community composition thatseem related to increased inundation frequency a sign that thewetland is not keeping pace with rising sea level (Perry andHershner 1999)

Wetlands will also respond to elevated levels of atmospheric CO2increasing temperatures and changing salinity patterns A Scirpusolneyi wetland sedge community of the Rhode River (a subestuary ofthe Chesapeake Bay) that was exposed to an approximate doublingof atmospheric CO2 over a 17-year period revealed enhanced shootdensity shoot biomass and rates of net CO2 uptake compared toambient exposures (Rasse et al 2005) In contrast Spartina patensshowed no significant response to CO2 (Erickson et al 2007) Rasseet al (2005) also clearly documented salinity stress on S olneyi withsignificant anti-correlations at the interannual time scale betweensalinity and the three growth measures referred to above ElevatedCO2-stimulation of plant growth has important implications forbrackish marshes many of which are dominated by C3 plant speciessuch as S olneyi Indeed recent results from a Rhode River marsh(Langley et al 2009) show that elevated CO2 increases root biomasswhich in turn raises elevation of the tidal marsh soil The increase inelevation was 39 mm yr1 similar to the current average rate ofrelative sea-level rise in the Chesapeake Bay (Section 25) Thuselevated CO2 may stimulate marsh accretion and ameliorate marshlosses projected from accelerated sea-level rise The combination oftemperature increases and elevated CO2 concentrations mayproduce different effects on marshes than elevated CO2 alone butwe presently know little about these interactions

The impact of rising sea level is compounded by on-going land-use and land-cover change and associated shoreline hardeningIncreased shoreline hardening has limited the ability of marshes tomigrate in response to sea-level rise Land-use projections for someportions of the Chesapeake Bay shoreline exist (eg Dingerson2005) but we know of none that simultaneously link to sea-level riseand shoreline condition However it seems likely that the threat ofincreased erosion and damage to property from higher sea levelmore intense storms and a greater tidal range (Section 25) will be anincrease in shoreline hardening and stabilization structures Thesestructures can reduce the sandy sediment source that is critical tohealthy shoreline habitat (National Research Council 2007)

Inundation of coastal wetlands by rising sea levels may stressthe systems in ways that enhance the potential for invasion of lessdesirable species such as Phragmites australis (one of six speciesidentified as causing or having the potential to cause significantdegradation of the aquatic ecosystem of the Bay) (US Environ-mental Protection Agency 2008a) Reported impacts includesignificant loss of plant diversity (eg Meyerson et al 2002)changes in marsh hydrology with the development of Phragmitesstands (see Marks et al 1994) and a reduction in insect avian andother animal assemblages (Chambers et al 1999) Shifts withinnative plant communities are also probable although difficult topredict with current experimental data (Dukes 2007) Additionallyfor many marsh systems to persist a continued input of suspendedsediment from inflowing streams and rivers is required to allow forsoil accretion Climate change will alter the timing and overalldelivery of sediment from upstream sources but these conse-quences remain uncertain (Section 31)

7 Fish and shellfish

Species in the Chesapeake Bay food web vary in the relativeimportance and specific optimal values of temperature photope-riod salinity and prey abundances as controls or cues for growthreproduction and migration As a result climate change that affectsthese variables or that decouples the current relationships among


them may differentially affect species within the food web Thelikely results will include direct effects on the timing and magni-tude of abundances of individual species as well as changes in thematch between seasonal abundances of some predators and prey

71 Temperature impacts on fish and shellfish

As warming progresses it should differentially affect the warm-and cold-temperate fish and shellfish that utilize the Bay eitheryear round or as seasonal feeding nursery and wintering habitatSpecies with more southerly distributions and temperature toler-ances will likely benefit (Austin 2002 Wood 2000) but some cold-temperate species near the upper limits of thermal tolerances mayreduce use of the Chesapeake Bay and have lower productionwithin Bay waters Thermal tolerances of embryos and early larvalstages may be particularly important in determining shifts inspecies distributions (Rombough 1997) Developing embryos offishes are generally less able to acclimate and compensate forchanges in temperature and tend to have narrower temperaturetolerances than other life stages

Northward range expansions and extended seasonal use of Baywaters by warm-water species may enhance some fisheries Shrimpsof the genus Farfantepenaeus which now support important fish-eries in North Carolina (Hettler 1992) could increase and supportviable fisheries in the Chesapeake Bay and elsewhere in the mid-Atlantic Bight Species with southerly distributions that alreadyoccur in Chesapeake Bay and Virginia coastal waters and would beexpected to increase in abundance or seasonal duration within Baywaters include southern flounder Paralichthys lethostigma cobiaRachycentron canadam spadefish Chaetodipterus faber Spanishmackerel Scomberomorus maculatus mullet Mugil curema tarponMegalops atlanticus and pinfish Lagodon rhomboides (Murdy et al1997) as well as sub-tropical drums (Sciaenidae) such as black drumPogonias cromis red drum Sciaenops ocellatus weakfish Cynoscionregalis spotted sea trout C nebulosus spot Leiostomus xanthurus andNorthern Menticirrhus saxatilis and Southern kingfish M americanus

Higher water temperatures during winter in particular may havepositive effects on some species Overwintering mortality can be animportant factor contributing to year-class strength (Conover andPresent 1990) Higher winter temperatures result in higher juvenileoverwintering survival and stronger year classes of Atlantic croakerMicropogonias undulatus in the mid-Atlantic (Hare and Able 2007)and increase overwintering survival of both juvenile (Bauer 2006)and adult (Rome et al 2005) blue crab in the Chesapeake Bay Milderwinters should also lead to longer growth seasons for species resi-dent to the Chesapeake Bay such as oysters blue crab eels whiteperch Morone americana and the resident portion of the striped bassMorone saxatilis population (Hurst and Conover 1998 Johnson andEvans 1996) Longer growth seasons could lead to increasedproductivity and yield of commercial fisheries by increasing annualgrowth rates and increasing the size or decreasing the age ofreproduction (Puckett et al 2008) This assumes that warmerwinters coincide with sufficient prey and that warming in otherseasons does not offset longer growth seasons due to increasedincidence of disease or hypoxia (Sections 54 and 74)

The degree to which the Chesapeake Bay freezes over is alreadymuch reduced in comparison to 50 years ago (Boesch 2008) Lackof surface freezing in shoreline habitats could increase opportuni-ties for oysters and other intertidal species to colonize shorelinesand form emergent reefs Shoreline and emergent reefs wouldincrease access to new habitat for restoration and aquaculture

In contrast to the generally positive effects predicted forsoutherly species higher temperatures may decrease the arealextent of bioenergetically favorable Bay habitats for cold-temperatespecies both directly and in combination with low oxygen (Sections

54 and 74) Species that are at their southernmost range in themid-Atlantic region will be eliminated from the Chesapeake Bay ifwater temperatures reach levels that are lethal or that inhibitsuccessful reproduction For example the commercially importantsoft clam Mya arenaria in the Chesapeake Bay is near its southerndistribution limit and may be extirpated if temperatures approachand remain near w32 C (Kennedy and Mihursky 1971) Temperatefish species such as yellow perch Perca fulvescens white perchstriped bass black sea bass Centropomis striata tautog Tautogaonitis summer Paralichthys dentatus and winter flounders Pleuro-nectes americanus silver hake Merluccius bilinearis and scup Sten-otomus chrysops will likely be stressed by Chesapeake warmingduring summer

Warmer and shorter winter seasons may nevertheless allow forearlier spring immigration and later fall emigration of some coastalspecies including cool-temperate fishes (eg see Frank et al 1990for the St Lawrence region) Assuming continued use of Bay watersstriped bass shads (Alosa sp) and other fish that migrate into theChesapeake for spring spawning will likely arrive earlier Americanshad now migrate up the Columbia River (where they have beenintroduced) 38 days earlier than during the 1950s as watertemperatures have increased as a result of reductions in spring flowby the Bonneville Dam (Quinn and Adams 1996)

Physical and ecological factors other than temperature maypreclude a smooth transition to a balanced ecosystem dominated bywarm-water fishery species Oligohaline-upper mesohaline species(such as the bivalves Mytilopsis leucophaeata or Ischadium recurvum)that live only in estuaries may spread northward slowly if theycannottolerate the marine conditions that occur between estuariesIncreasing temperatures along with other climate-related changes inthe Bay environment may also facilitate the successful northwardexpansion of non-native species (Stachowicz et al 2002) and path-ogens (Cook et al1998) As recently as 1987 the Chesapeake Bay wasthe largest oyster producer on the Atlantic and Gulf of Mexico coasts(Haven1987) While overfishing has historically played an importantrole in the demise of this fishery two oyster pathogens Perkinsusmarinus (Dermo) and Haplosporidium nelsoni (MSX) have contrib-uted to the long-term decline and have hindered the populationrsquosrecovery despite considerable restoration efforts (Andrews 1996)Warmer winters appear to have already increased these diseases inoyster populations in Atlantic Coast estuaries (eg Cook et al 1998)The strong temperature dependence of Dermo inparticular suggeststhat the Chesapeake region could experience increased oyster para-site stress in subtidal oysters as local water temperatures increase

Some fish parasites might also benefit from warmer climesWeisberg et al (1986) have documented increases in the interme-diate host Limnodrilus sp for the fish redworm Eustrongylides sp(pathogenic to avian definitive hosts) in warming eutrophic watersof the Bay the fish infected include yellow perch (Muzzall 1999)which is common to the Bay Warmer shelf waters might also leadto earlier arrivals and later departures of pelagic fishes (eg Franket al 1990) favoring transmission of pelagic oriented parasites ashas been suggested for the St Lawrence River and Japan increasinghuman illness from pathogen transfer via undercooked fish (Hubertet al 1989)

Winter survival of potential pathogens is also hinted at in recentobservations in upper river basins In the last four years winterwater temperatures have been substantially higher in the upperShenandoah River area than the past and each spring thereaftermajor smallmouth bass mortalities have been observed Winterpathogen survival has been suggested as one explanation for theserecurring events (Chesapeake Bay Foundation 2007) and if thishypothesis is correct overwintering success and subsequent springillnesses or mortalities may become increasingly common asregional water temperatures rise


Finally warming may also influence pollutant impacts Highertemperature-induced-mercury methylation (Booth and Zeller2005) has for example been suggested as a possible mechanism toincrease mercury uptake in fish and increase potential fetal impacts(Bambrick and Kjellstrom 2004 from McMichael et al 2006) fishtissue mercury concentrations are already a public health concernin Chesapeake jurisdictions

72 Salinity impacts on fish and shellfish

The most pronounced effects of altered salinity distributions(Section 42) on fishery species will likely result from changes in thedistribution and abundance of predators prey and pathogens Thetwo examples considered here the eastern oyster and gelatinouszooplankton illustrate how these factors may play out

Salinity affects the eastern oyster in at least three ways First theoyster has a physiological salinity range of 5 to 35 Secondmortality from Perkinsus and Haplosporidium infections is greatestat salinities above about 12 (Haven 1987) Third spatfall success(recruitment) in the Bay oyster population has been shown to bepositively affected by higher salinity (eg Kimmel and Newell2007) The net effect of these three factors in the face of salinityincreases which will very likely occur if precipitation remainsunchanged may depend on whether the combination of favorableconditions for recruitment and high parasite stress affect selectionfor disease tolerance in infected oysters

The second example involves the two dominant gelatinouszooplankton species within the Bay the ctenophore Mnemiopsisleidyi and its scyphomedusan predator Chrysaora quinquecirrha(the sea nettle) (Purcell and Arai 2000) Both of these species feeddirectly on fish eggs and larvae (eg Cowan and Houde 1993Monteleone and Duguay 2003) as well as on zooplankton that areimportant prey for adult forage fish and other fish species in earlylife stages (eg Burrell and Van Engel 1976 Purcell 1992) Mne-miopsis leidyi has a greater ability to deplete its prey than doesC quinquecirrha and also feeds on oyster larvae Interannualvariability in salinity and flow strongly affect the timing of peak seanettle abundances with abundances peaking earlier in years ofabove-average salinity (Breitburg and Fulford 2006) Conse-quently climate change may ultimately influence the timing andmagnitude of direct effects of sea nettle consumption of icthyo-plankton and other zooplankton as well as indirect effects medi-ated through the control sea nettles exert over their ctenophoreM leidyi prey

73 Prey production impacts on fish and shellfish

The predicted combination of increases in both temperatures andwinter flows could decouple the combination of environmentalfactors historically associated with favoring particular Bay speciesand high total fisheries production of Chesapeake Bay waters Anal-ysis of Chesapeake assemblages show that low temperatures andhigh flows in winter are associated with high summer-fall abun-dances of juvenile Atlantic silversides Menidia menidia (an importantforage fish) striped bass white perch and Atlantic needlefishStrongylura marina (Kaushal et al in press) Species associated withthe converse low winter flows and high winter temperaturesinclude bluefish Pomatomus saltatrix spot bay anchovy Anchoamitchilli and northern puffer Sphoeroides maculatus Shifts betweenthese two juvenile assemblages occurred between average wintersthat differed by w1 Cdwell within the range of warming expectedin the next 50 years (Section 22)dbut effects of higher temperaturescombined with higher flows is difficult to predict

Similarly the predicted earlier and stronger spring freshet(Section 24) on Chesapeake Bay fisheries is not clear Fisheries

production in the Bay as in most mid-latitude temperate systems isstrongly tied to the progression of annual production that is initi-ated by high early-spring streamflow (Section 51) (eg Silvert1993) The timing and magnitude of the spring zooplankton bloomthat provides food for young-of-the-year of spring spawning fishesand forage fish species that actively feed in the Bay in early spring isinfluenced by winter weather and spring streamflow (Kimmel et al2006) A change in the timing of the spring freshet could alterfishery production but specific effects are better known for reducedand later freshets (Wood and Austin 2009) than for changes in thedirection predicted

Warmer summers may also affect predation mortality of early lifestages of summer breeding fish and shellfish Lethal temperature forthe lobate ctenophore M leidyi collected from the Chesapeake Bay isapproximately 30 C in laboratory experiments (Breitburg 2002)Both latitudinal variation and shifts in the timing of peak ctenophoreabundances with increasing temperatures in Narragannsett Bay(Sullivan et al 2001) indicate the potential for temperature increasesto alter the temporal overlap between these predators and the earlylife stages of fish and shellfish on which they prey

74 Dissolved oxygen impacts on fish and shellfish

Low dissolved oxygen affects growth mortality distributionsand food web interactions of a wide range of organisms in theChesapeake Bay (eg Breitburg et al 2003 Kemp et al 2005)Seasonal hypoxia results in mortality of benthic animals in thedeeper parts of the Bay such that deep benthic macrofauna areessentially absent in the summer and depauperate during othertimes of the year (eg Sagasti et al 2001) Mortality of animals canalso occur in shallow water environments with episodic advectionof hypoxic or anoxic bottom water shoreward (Breitburg 1990) andwhere warm calm conditions result in diel hypoxic events inshallow waters (Tyler and Targett 2007)

In addition to increasing mortality directly hypoxia may havestrong effects on the ecosystem and its fisheries through behav-ioral and physiological responses of organisms that alter trophicinteractions over broad time and space scales (Breitburg et al2001) For example increases in summer temperatures andincreased anoxia or hypoxia may exclude species such as stripedbass and Atlantic sturgeon (Acipenser oxyrhynchus) from benthicfeeding grounds and bioenergetically favorable cool deep-waterenvironments (Coutant 1985 Secor and Gunderson 1998) Ina simulation on the combined effects of hypoxia and temperatureNiklitschek and Secor (2005) observed that even a small overallwarming (1 C) during summer months could virtually eliminatesuitable habitats for juvenile sturgeons (Fig 11) Low dissolvedoxygen can also alter trophic interactions that support fisheryspecies by inhibiting production of ecologically importantzooplankton grazers (Roman et al 1993) increasing some speciesrsquosusceptibility to predation (eg Breitburg et al 1997) andproviding predatory refuge to others (Sagasti et al 2001)Repeated exposure of deeper subtidal oyster populations off Cal-vert Cliffs Maryland to low-oxygen bottom water resulted indepressed growth rates relative to rates noted for oysters in shal-lower depths where exposure to low-oxygen water was lessfrequent (Osman and Abbe 1994)

Warming will increase the extent and severity of effects ofhypoxia on macrofauna by affecting dissolved O2 concentrations asdiscussed in Section 54 but also by increasing the oxygenrequirements of fishes (eg Shimps et al 2005) The combinedeffect is very likely to be a further reduction in the quality andspatial extent of suitable habitat in the Chesapeake Bay system fora wide range of aerobic organisms

000003 000053 000004 000054 000003 000053 000004 000054

0400-

0300-

0200-

0100-

0000

0100

0200

0300

0400

a b

)81 MTU( edutignoL

0000014

0000534

0000034

0000524

0000024

0000514

Latit

ude

(UTM

18)

op suoenatnatsnI noitcudorp laitnet

Fig 11 Habitat suitability maps for young-of-the-year Atlantic sturgeon in bottom waters of the Chesapeake Bay during July Habitat suitability is indexed as instantaneous dailycohort production rates (a) mean for the 1991ndash2000 period (b) result of 1 C increase of mean temperature for the same period Figure modified from Niklitschek and Secor (2005)


75 Other impacts on fish and shellfish

Among the greatest concerns about climate change effects onfish and shellfish is the consequence of sea-level rise on tidalwetlands (Section 62) Reductions in tidal marsh and submersedvegetation directly affect the Bayrsquos fisheries because many fishesand crustaceans utilize these habitats as nursery areas and foraginggrounds (eg Boesch and Turner 1984 Kneib and Wagner 1994)Ecologically and economically important species that utilize thesehabitats include forage fishes such as mummichog (Fundulus het-eroclitus) eastern mosquitofish (Gambusia holbrooki) and preda-tory nekton such as summer flounder spotted seatrout stripedbass and blue crabs Because many of these species spend much oftheir life spans in the coastal Atlantic significant loss or degrada-tion of these habitats could also affect the larger-scale NortheastUS continental shelf large marine ecosystem

There is an indication that intense storms in the Chesapeake Bayregion may increase in the future (Section 26) and the impacts onfisheries could be dramatic For example strong winds associatedwith storms influence how larval fish and crabs move into and out ofthe Chesapeake Bay Summer and fall tropical storms favor thedispersal of larval Atlantic croaker into the Chesapeake Bay resultingin higher juvenile counts during the subsequent winter andsummer (Houde et al 2005 Montane and Austin 2005) Blue crabsspawn in coastal waters adjacent to the mouth of the Chesapeakeand depend upon northward and westward winds to transport theiroffspring back into the Chesapeake (Olmi 1995) Such transport canbe aided or curtailed by tropical storm activity For similar reasonswinter storm activity can influence dispersal into the Chesapeakeand subsequent nursery habitat use by coastal spawning fish such asmenhaden and spot (Epifanio and Garvine 2001)

Bivalves as well as a number of other organisms such as fora-minifera rely on pH-sensitive processes to build calcium carbonateshells and other structures Therefore CO2 increases coulddramatically alter calcification in these animals (Gazeau et al2007) Consistent with this pattern Miller et al (2009) found thatChesapeake Bay eastern oyster larvae reared in experimental

aquaria under atmospheric CO2 levels that could be reached thiscentury (560 ppm and 800 ppm Fig 2b) grew and calcified moreslowly than under ambient atmospheric conditions whentemperature salinity light level daynight cycle and food qualityquantity were held constant

8 Discussion and summary

The overall picture that emerges from the above review is thatclimate variability has influenced a multitude of physical chemicaland biological processes in the Chesapeake Bay and that futurechanges in climate will profoundly influence the Bay Uncertaintyvaries dramatically however among the various potential impactsbecause (1) the uncertainty in future climate forcing of the Ches-apeake Bay region varies dramatically among the proximate forcingagents (atmospheric CO2 water temperature sea level andstreamflow) and (2) the uncertainty in the sensitivity of Bay systemcomponents to climate varies dramatically among the forcingagents and the components In the remainder of this section wesummarize projections of climate change in the Bay region andtheir impacts on the Bay emphasizing the degree of certainty andthus prioritizing future research needs

With regard to climate forcing agents much greater certaintyexists for projected trends in atmospheric CO2 water temperatureand sea level (all virtually certain to increase) than for storminess(likely to increase) and annual streamflow (about as likely toincrease as not) However it does appear likely that winter andspring streamflow will increase Further heat waves and precipi-tation intensity are likely to increase which will likely result ingreater extremes (high and low) of streamflow The greatestresearch needs are thus for improved precipitation and streamflowprojections and for robust error bounds on these and otherprojections (eg Tebaldi et al 2005) A better understanding is alsorequired of the causes of 20th-century changes in precipitation andtemperature in the Bay watershed

With regard to the biogeochemistry of the Bay watersheda fundamental problem is the lack of a mechanistic understanding


of nutrient cycling on the watershed scale which is demonstratedby the current controversy about temperature vs precipitationimpacts on nutrient fluxes averaged over many years (Howarthet al 2006 Schaefer and Alber 2007) It is likely that nutrient andsediment loading during winter and spring will increase becauseflow will likely increase during this time Also given no change inthe annual flow regime it is likely that phosphorus and sedimentloading will increase as a result of the more intense and potentiallyless frequent rain events However a quantitative relationshipbetween particle loading and precipitation intensity remains to beestablished Over a longer time period changes in the land use andland cover across the Bay watershed (caused in part by climatechange) may dominate the change in flux Research is needed tobetter quantify current material budgets (such as reducing uncer-tainty in atmospheric N deposition estimates) and to developrobust mechanistic and quantitative models of large-scale water-shed nutrient and sediment cycling (with some species-specificrepresentation) that can be applied over a variety of time scales(daily to decadal) Of particular importance is an improved under-standing of the combined impacts of climate and land-use andland-cover change including climate impacts on human infra-structure (eg storm water management systems or more specu-latively concentrated animal feeding operations)

Despite all of the research on the physical oceanography of theChesapeake Bay little is known about its seasonal and interannualcharacteristics the time scales most relevant for climate changeSummertime stratification and landward advection below thepycnocline are likely to increase in response to increases in winter-spring streamflow but other circulation responses to climate suchas those due to changes in winds and sea level are poorly knownbecause of the uncertainty in the climate change itself as well as thelack of research on the response of estuarine physics to climatechange Salinity will very likely increase in response to sea-levelrise and warming alone (due to increased evapotranspiration andthus decreased streamflow) but the lack of consensus in annualprecipitation changes makes the overall direction of salinity changehighly uncertain Increases in salinity variability are possible on theseasonal time scale (if summers do not get wetter) and are likely onthe interannual time scale (due to droughts) The relationship ofsediment loading to flow is well constrained on annual time scalesbut not for extreme events The connection between other sedi-ment sources and climate is poorly known Predictive modeling ofextreme temperature events which are important for SAV andlikely other organisms is lacking as is predictive modeling of windpatterns directions and strengths which are important for holo-and meroplankton advection but also summer productivitymaxima A dedicated research program combining observationsand models is needed for a better understanding of these complexestuarine physical processes under the changing conditions antic-ipated for the next century

Current research suggests that climate-induced increases inwinter and spring nutrient loading to the Bay will likely result inincreasing phytoplankton production Combined with highertemperatures that promote decreased oxygen solubility and greaterheterotrophy this increase in phytoplankton production will likelylead to more intense and more frequent episodes of hypoxia Highertemperatures and CO2 levels appear likely to select for increases inharmful algal blooms Though the direction of many trends is fairlycertain quantitative relationships are few particularly fortemperature impacts which at interannual time scales have beenoverwhelmed in many cases by streamflow impacts Thus we havevery few whole-system views of the response of Bay biogeo-chemistry to temperature This is unfortunate because estuarinebiogeochemical processes are complex and highly non-linear andefforts to untangle the biogeochemical impacts of multiple climate

forcings (temperature CO2 streamflow sea level) from each otherand from other forcing agents (eg land-use and land coverchange) will require long-term attention and dedicated resources

Field observations and experiments have illustrated the highsensitivity of Bay vascular plants to temperature salinity light CO2and sea level and the complexity of the interactions among theseforcing agents is just beginning to be understood For example it isclear that eelgrass is likely to respond positively to the direct effectsof higher atmospheric CO2 levels but not if water clarity is unim-proved or is further degraded by the likely increases in precipita-tion intensity Similar to SAV estuarine wetlands will respondpositively to higher CO2 levels but this response may be over-whelmed by inundation due to sea-level rise and changes in landuse and land cover In summary research is needed to capture theimplications of multiple interacting stressors that will influence thedistribution and abundance of Bay vascular plants in the future

While some consequences of climate change on fish and shellfishmay be positive (such as increases in species with more southerlydistributions) it appears that most impacts (such as increasedhypoxia and CO2) will be negative However there is great uncer-tainty in the response of higher trophic levels in the Bay to climatechange because of the accumulated uncertainty in projected changesin climate watershed hydrology and biogeochemistry lower trophiclevels and pathogens Further compounding this uncertainty is thehigh non-linearity inherent in trophic interactions and faunal lifecycles As with other Bay living resources whole-system approachesinvolving monitoring process studies and numerical modeling willbe required to develop a quantitative understanding of the impactsof climate change on fish and shellfish

Climate change in the Chesapeake Bay region has the potentialto create cultural and socio-economic impacts on a wide range ofstakeholders including commercial watermen farmers propertyowners and municipal and county governments To date nosystematic research has been undertaken to investigate howclimate change will impact cultural and socio-economic processesand vice versa across the Bay region The hydrologic biogeochem-ical physical and living resources impacts described in this reviewpresent a number of areas where livelihoods may be affected andthere is the potential for a wide range of social and economicimpacts particularly on weather-dependent industries and entitiessubject to air or water quality regulation

An important component of any social science research agendaon climate change will be the assessment of local knowledge andperceptions of climate change This is because local knowledge maybe able to identify impacts that are occurring long before theseimpacts are noticed by the scientific community and local knowl-edge can be used to extend the reach of scientific inquiry intoanalysis of impacts on livelihoods communities and land usepractices Also local populations will have cultural perceptions ofclimate change that may not match well with the models of climatechange and impacts deployed by scientists and policymakers thusraising the possibility that information produced by science andpolicy may not be effective in changing behaviors or alleviatingimpacts related to climate change goals

In summary the Chesapeake Bay system has shown a highsensitivity to climate variability and is thus likely given currentclimate projections to behave very differently in the future even ifother important influences (eg land use change and fisheries)remain unchanged The majority of climate change impacts arenegative and thus climate change presents a serious challenge tocurrent efforts to restore the Chesapeake Bay However there isa wide divergence in possible climate futures (perhaps even widerthan shown in Fig 2) and thus we emphasize that although it isvery likely that climate change impacts will occur it is also clearthat the severity of these impacts are directly under human control


Efforts to reduce CO2 emissions now will thus reduce future climatechange impacts on the Chesapeake Bay and other estuaries

We recommend a common strategy for addressing the gaps inunderstanding about Bay physical processes biogeochemistry livingresources and human systems a coordinated program of long-termresearch based on a whole-system approach linking monitoringprocess studies and numerical modeling Such a program can andshould be designed around specific near- and long-term researchobjectives and designed to yield fundamental improvements inunderstanding of these systems The scope of the issues facing theChesapeake Bay requires that the recommended program reflecta diverse interdisciplinary multi-institution approach with a foun-dation of new dedicated resources and strong institutional rela-tionships between Bay managers and decision makers

Other estuaries will respond differently to climate change thanthe Chesapeake Bay and so our findings cannot be directly appliedto other systems There are three main reasons for this Firstestuaries themselves have a tremendous variety in their physicalchemical and biological characteristics these characteristics rangefor example from well mixed to stratified from light-limited tonutrient limited and from well oxygenated to seasonally anoxicSecond estuaries vary dramatically in the ways and degree towhich they have been influenced by human activity such asnutrient over-enrichment changes in sediment loading shorelinehardening and overfishing These two factors characterize thecurrent state of an estuary and given the highly non-linear way inwhich estuaries respond to external forcing (Kemp et al 2005Kemp and Goldman 2008) their response to climate change willvery likely depend strongly on their initial state A dramaticexample of this is comparison of the partially stratified nutrient-limited Chesapeake with its neighbor to the north the DelawareBay which is well mixed and light-limited Whereas projectedwinterndashspring streamflow increases into the Chesapeake wouldlikely lead to a stronger spring phytoplankton bloom and greatersummertime anoxia we might expect that such a streamflowchange would decrease phytoplankton production and have littleor no effect on oxygen levels in the Delaware Bay

The third reason that other estuaries will respond differently toclimate change than the Chesapeake Bay will is that the climatechange itself will differ though with differing degrees among theforcing agents For example while all estuaries will experienceessentially the same higher levels of atmospheric CO2 in the futurethe degree of warming and sea-level rise will vary regionally andseasonally More importantly however streamflow changes willvary dramatically with region in terms of magnitude and timingSubtropical estuaries will very likely see decreases in annualstreamflow due to the combined impact of decreased precipitationand higher evapotranspiration Subpolar estuaries will see reduc-tions in spring streamflow as a result of decreases in snowfall andearlier melting of snow on land

The main aspect of the current study can be generalized to otherestuaries is the approach which may be the most novel feature of ourwork There is a diverse body of research on the impacts of climatechange on estuaries but very little of itdincluding aspects related toclimate change itself and implications for watershed processesestuarine circulation biogeochemistry vascular plants and fish-eriesdhas been synthesized for one system to create a holisticpicture of the impacts and place them in the context of other stressors(eg land-use change) and restoration efforts This work was madepossible by bringing together scientists with regional expertise ina wide diversity of environmental sciences including climateforestry hydrology biogeochemistry wetland ecology hydrody-namics plankton ecology fisheries and humanenvironment inter-actions Efforts such as this in other estuaries followed by cross-

comparisons among these estuaries may be the most fruitful path toa generalized theory of the impacts of climate change on estuaries

Acknowledgments

We are grateful for input and encouragement from the Scienceand Technical Advisory Committee of the Chesapeake Bay ProgramCritical comments from M Castro S Crate T Johnson D Kimmeland J Shen on an early version of this paper are appreciated Twoanonymous reviewers were helpful in improving the manuscript

References

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Adolf JE Yeager CL Miller WD Mallonee ME Harding LW 2006 Environ-mental forcing of phytoplankton floral composition biomass and primaryproductivity in Chesapeake Bay USA Estuarine Coastal and Shelf Science 67108ndash122

Allard J Keim BD 2007 Spuriously induced temperature trends in the SoutheastUnited States Theoretical and Applied Climatology 88 103ndash110

Andrews JD 1996 History of Perkinsus marinus a pathogen of oysters in Ches-apeake Bay 1950ndash1984 Journal of Shellfish Research 15 13ndash16

Austin HM 2002 Decadal oscillations and regime shifts A characterization of theChesapeake Bay marine climate American Fisheries Society Symposium 32155ndash170

Bambrick HJ Kjellstrom TE 2004 Good for your heart but bad for your babyRevised guidelines for fish consumption in pregnancy The Medical Journal ofAustralia 181 61ndash62

Bauer LJ 2006 Winter mortality of the blue crab (Callinectes sapidus) in theChesapeake Bay Masters thesis University of Maryland

Beer S Koch E1996 Photosynthesis of marine macroalgae and seagrasses in globallychanging CO2 environments Marine Ecology Progress Series 141 199ndash204

Bintz JC Nixon SW Buckley BA Granger SL 2003 Impacts of temperature andnutrients on coastal lagoon plant communities Estuaries and Coasts 26 765ndash776

Boesch DF (editor) 2008 Global Warming and the Free State ComprehensiveAssessment of Climate Change Impacts in Maryland Report of the Scientific andTechnical Working Group of the Maryland Commission on Climate ChangeUniversity of Maryland Center for Environmental Science Cambridge Maryland

Boesch DF Turner RE 1984 Dependence of fishery species on salt marshes therole of food and refuge Estuaries and Coasts 7 460ndash468

Booth S Zeller D 2005 Mercury food webs and marine mammals implicationsof diet and climate change for human health Environmental Health Perspec-tives 113 521ndash526

Boyer EW Howarth RW 2008 Nitrogen fluxes from rivers to the coastal oceansIn Capone DG Bronk D Mulholland M Carpenter EJ (Eds) Nitrogen in theMarine Environment second ed Academic Press San Diego pp 1565ndash1587

Boynton WR Kemp WM 2000 Influence of river flow and nutrient loads onselected ecosystem processes a synthesis of Chesapeake Bay data In Hob-bie JE (Ed) Estuarine Science A Synthetic Approach to Research and PracticeIsland Press Washington DC pp 269ndash298

Breitburg DL 1990 Near-shore hypoxia in the Chesapeake Bay patterns andrelationships among physical factors Estuarine Coastal and Shelf Science 30593ndash609

Breitburg DL 2002 Effects of hypoxia and the balance between hypoxia andenrichment on coastal fishes and fisheries Estuaries 25 767ndash781

Breitburg DL Adamack A Rose KA Kolesar SE Decker B Purcell JEKeister JE Cowan JH 2003 The pattern and influence of low dissolvedoxygen in the Patuxent River a seasonally hypoxic estuary Estuaries and Coasts26 280ndash297

Breitburg DL Fulford RS 2006 Oyster-sea nettle interdependence and alteredcontrol within the Chesapeake Bay ecosystem Estuaries and Coasts 29 776ndash784

Breitburg DL Loher T Pacey CA Gerstein A 1997 Varying effects of low dis-solved oxygen on trophic interactions in an estuarine food web EcologicalMonographs 67 489ndash507

Breitburg DL Pihl L Kolesar SE 2001 Effects of low dissolved oxygen on thebehavior ecology and harvest of fishes a comparison of the Chesapeake Bayand Baltic-Kattegat systems In Rabalais NN Turner RE (Eds) CoastalHypoxia Consequences for Living Resources and Ecosystems AmericanGeophysical Union Washington DC pp 241ndash267

Brooks RP Wardrop DH Bishop JA 2004 Assessing wetland condition ona watershed basis in the Mid-Atlantic Region using synoptic land-cover mapsEnvironmental Monitoring and Assessment 94 9ndash22

Brookshire ENJ Valett HM Thomas SA Webster JR 2007 Atmospheric N depo-sition increases organic N loss from temperate forests Ecosystems 10 252ndash262


Burkett V Kusler J 2000 Climate change Potential impacts and interactions inwetlands of the United States Journal of the American Water Resources Asso-ciation 36 313ndash320

Burrell VG Van Engel WA 1976 Predation by and distribution of a ctenophoreMnemiopsis leidyi (A Agassiz) in the York River estuary Estuarine and CoastalMarine Science 4 235ndash242

Castro MS Driscoll CT Jordan TE Reay WG Boynton WR 2003 Sources ofnitrogen to estuaries in the United States Estuaries and Coasts 26 803ndash814

Cerco CF Noel MR Linker L 2004 Managing for water clarity in ChesapeakeBay Journal of Environmental Engineering 130 631ndash642

Chambers RM Meyerson LA Saltonstall K 1999 Expansion of Phragmites aus-tralis into tidal wetlands of North America Aquatic Botany 64 261ndash273

Chesapeake Bay Foundation 2007 Climate Change and the Chesapeake BayChallenges Impacts and the Multiple Benefits of Agricultural ConservationWork Chesapeake Bay Foundation Annapolis MD 18 pp

Chesapeake Bay Program 2003 Bay Program partners adopt new innovativeapproach to restoring water quality in Chesapeake Bay Press Release April 152003 Annapolis MD

Christensen JH Hewitson B Busuioc A Chen A Gao X Held I Jones RKolli RK Kwon WT Laprise R Rueda VM Mearns L Menendez CGRaisanen J Rinke A Sarr A Whetton P 2007 Regional climateprojections In Solomon S Qin D Manning M Chen Z Marquis MAveryt KB Tignor M Miller HL (Eds) Climate Change 2007 ThePhysical Science Basis Contribution of Working Group I to the FourthAssessment Report of the Intergovernmental Panel on Climate ChangeCambridge University Press Cambridge United Kingdom and New YorkNY USA pp 847ndash940

Church JA White NJ Coleman R Lambeck K Mitrovica JX 2004 Estimates ofthe regional distribution of sea level rise over the 1950ndash2000 period Journal ofClimate 17 2609ndash2625

Colwell RR 1996 Global climate and infectious disease the cholera paradigmScience 274 2025ndash2031

Conover DO Present TMC 1990 Countergradient variation in growth ratecompensation for length of the growing season among Atlantic silversides fromdifferent latitudes Oecologia 83 316ndash324

Cook T Folli M Klinck J Ford S Miller J 1998 The relationship betweenincreasing sea-surface temperature and the northward spread of Perkinsusmarinus (Dermo) disease epizootics in oysters Estuarine Coastal and ShelfScience 46 587ndash597

Coutant CC 1985 Striped Bass temperature and dissolved oxygen a speculativehypothesis for environmental risk Transactions of the American Fisheries Society114 31ndash61

Cowan Jr JH Houde ED 1993 Relative predation potentials of scyphomedusaectenophores and planktivorous fish on ichthyoplankton in Chesapeake BayMarine Ecology Progress Series 95 55ndash65

Cronin TM Dwyer GS Kamiya T Schwede S Willard DA 2003 MedievalWarm Period Little Ice Age and 20th century temperature variability fromChesapeake Bay Global and Planetary Change 36 17ndash29

Davis A 2007 Stormwater Impacts of Growth on Water Quality Scientific SummitAnnapolis MD

Davis JL Mitrovica JX 1996 Glacial isostatic adjustment and the anomalous tidegauge record of eastern North America Nature 379 331ndash333

Dingerson LM 2005 Predicting future shoreline condition based on land usetrends logistic regression and fuzzy logic Masters thesis Virginia Institute ofMarine Science The College of William and Mary

Dukes JS 2007 Tomorrowrsquos plant communities different but how New Phy-tologist 176 235ndash237

Edwards M Richardson AJ 2004 Impact of climate change on marine pelagicphenology and trophic mismatch Nature 430 881ndash884

Ehlers A Worm B Reusch TBH 2008 Importance of genetic diversity in eelgrassZostera marina for its resilience to global warming Marine Ecology ProgressSeries 355 1ndash7

Epifanio CE Garvine RW 2001 Larval transport on the Atlantic continental shelfof North America a review Estuarine Coastal and Shelf Science 52 51ndash77

Erickson JE Megonigal JP Peresta G Drake BG 2007 Salinity and sea levelmediate elevated CO2 effects on C3ndashC4 plant interactions and tissue nitrogen ina Chesapeake Bay tidal wetland Global Change Biology 13 202ndash215

Eshleman KN Gardner RH Seagle SW Castro NM Fiscus DA Webb JRGalloway JN Deviney FA Herlihy AT 2000 Effects of disturbance onnitrogen export from forested lands of the Chesapeake Bay Watershed Envi-ronmental Monitoring and Assessment 63 187ndash197

Field JC Boesch DF Scavia D Buddemeier R Burkett VR Cayan DFogarty M Harwell M Howarth R Mason C 2001 Potential consequencesof climate variability and change on coastal areas and marine resources InNational Assessment Synthesis Team (Ed) Climate Change Impacts on theUnited States The Potential Consequences of Climate Variability and ChangeReport for the US Global Change Research Program Cambridge University PressCambridge UK pp 461ndash487

Frank KT Perry R Drinkwater KF 1990 Predicted response of NorthwestAtlantic invertebrate and fish stocks to CO2-induced climate change Trans-actions of the American Fisheries Society 119 353ndash365

Fu FX Warner ME Zhang Y Feng Y Hutchins DA 2008a Effects of increasedtemperature and CO2 on photosynthesis growth and elemental ratios inmarine Synechococcous and Prochlorococcus (Cyanobacteria) Journal ofPhycology 43 485ndash496

Fu FX Zhang Y Warner ME Feng Y Sun J Hutchins DA 2008b A comparisonof future increased CO2 and temperature effects on sympatric Heterosigmaakashiwo and Prorocentrum minimum Harmful Algae 7 76ndash90

Gazeau F Quiblier C Jansen JM Gattuso JP Middelburg JJ Heip CHR 2007Impact of elevated CO2 on shellfish calcification Geophysical Research Letters34 L07603 doi1010292006GL028554

Gibson JR Najjar RG 2000 The response of Chesapeake Bay salinity to climate-induced changes in streamflow Limnology and Oceanography 45 1764ndash1772

Gitay H Brown S Easterling W Jallow B 2001 Chapter 5 ecosystems and theirgoods and services In McCarthy JJ Canziani OF Leary NA Dokken DJWhite KS (Eds) Climate Change 2001 Impacts Adaptation and VulnerabilityCambridge University Press Cambridge UK pp 235ndash342

Gray DR 2004 The gypsy moth life stage model landscape-wide estimates ofgypsy moth establishment using a multi-generational phenology modelEcological Modelling 176 155ndash171

Greve TM Borum J Pedersen O 2003 Meristematic oxygen variability ineelgrass (Zostera marina) Limnology and Oceanography 48 210ndash216

Groisman PY Knight RW Karl TR 2001 Heavy precipitation and highstreamflow in the contiguous United States Trends in the twentieth centuryBulletin of the American Meteorological Society 82 219ndash246

Groisman PY Knight RW Karl TR Easterling DR Sun BM Lawrimore JH2004 Contemporary changes of the hydrological cycle over the contiguousUnited States trends derived from in situ observations Journal of Hydrome-teorology 5 64ndash85

Hagy JD 2002 Eutrophication hypoxia and trophic transfer efficiency in Ches-apeake Bay PhD thesis University of Maryland

Hagy JD Boynton WR Keefe CW Wood KV 2004 Hypoxia in Chesapeake Bay1950ndash2001 Long-term change in relation to nutrient loading and river flowEstuaries 27 634ndash658

Harding LW 1994 Long-term trends in the distribution of phytoplankton inChesapeake Bay roles of light nutrients and streamflow Marine EcologyProgress Series 104 267ndash291

Hare JA Able KW 2007 Mechanistic links between climate and fisheries alongthe east coast of the United States explaining population outbursts of Atlanticcroaker (Micropogonias undulatus) Fisheries Oceanography 16 31ndash45

Harvey HR OHara SCM Eglinton G Corner EDS 1989 The comparative fate ofdinosterol and cholesterol in copepod feeding implications for a conservativemolecular biomarker in the marine water column Organic Geochemistry 14635ndash641

Haven D 1987 The American oyster Crassostrea virginica in Chesapeake Bay InMajumdar SK Hall LWJ Austin HM (Eds) Contaminant Problems andManagement of Living Chesapeake Bay Resources The Pennsylvania Academyof Sciences Philadelphia pp 165ndash176

Hayhoe K Wake CP Huntington TG Luo LF Schwartz MD Sheffield JWood E Anderson B Bradbury J DeGaetano A Troy TJ Wolfe D 2007Past and future changes in climate and hydrological indicators in the USNortheast Climate Dynamics 28 381ndash407

Hettler WF 1992 Correlation of winter temperature and landings of pink shrimpPenaeus duorarum in North Carolina Fishery Bulletin 90 405ndash406

Hilton TW Najjar RG Zhong L Li M 2008 Is there a signal of sea-level rise inChesapeake Bay salinity Journal of Geophysical Research 113 C09002doi1010292007JC004247

Hirsch ME DeGaetano AT Colucci SJ 2001 An east coast winter stormclimatology Journal of Climate 14 882ndash899

Hirschberg DJ Schubel JR 1979 Recent geochemical history of flood deposits inthe Northern Chesapeake Bay Estuarine and Coastal Marine Science 9 771ndash784

Houde ED Bichy J Jung S 2005 Effects of hurricanes on Atlantic croaker(Micropogonias undulates) recruitment to Chesapeake Bay In Sellner KG (Ed)Hurricane Isabel in Perspective Chesapeake Research Consortium EdgewaterMaryland pp 5ndash160

Houghton JT Ding Y Griggs DJ Noguer M van der Linden PJ Dai XMaskell K Johnson CA (Eds) 2001 Climate Change 2001 The Scientific BasisContribution of Working Group I to the Third Assessment Report of the Inter-governmental Panel on Climate Change Cambridge University Press Cam-bridge United Kingdom and New York NY USA 881 pp

Howarth RW 2006 Atmospheric deposition and nitrogen pollution in coastalmarine ecosystems In Visgilio GR Whitelaw DM (Eds) Acid in the Envi-ronment Lessons Learned and Future Prospects Springer New York pp 97ndash116

Howarth RW Swaney DP Boyer EW Marino R Jaworski N Goodale C 2006The influence of climate on average nitrogen export from large watersheds inthe Northeastern United States Biogeochemistry 79 163ndash186

Hubert B Bacou J Belveze H 1989 Epidemiology of human anisakiasis inci-dence and sources in France The American Journal of Tropical Medicine andHygiene 40 301ndash303

Hurst TP Conover DO 1998 Winter mortality of young-of-the-year Hudson Riverstriped bass (Morone saxatilis) size-dependent patterns and effects onrecruitment Canadian Journal of Fisheries and Aquatic Sciences 55 1122ndash1130

IPCC 2007 Summary for policymakers In Solomon S Qin D Manning MChen Z Marquis M Averyt KB Tignor M Miller HL (Eds) Climate Change2007 The Physical Science Basis Contribution of Working Group I to the FourthAssessment Report of the Intergovernmental Panel on Climate Change Cam-bridge University Press Cambridge United Kingdom and New York NY USA

Iverson LR Prasad AM Schwartz MW 2005 Predicting potential changes insuitable habitat and distribution by 2100 for tree species of the Eastern UnitedStates Journal of Agricultural Meteorology 61 29ndash37


Johnson T Kittle J 2007 Sensitivity analysis as a guide for assessing andmanaging the impacts of climate change on water resources Water ResourcesIMPACT 8 17ndash19

Johnson TB Evans DO 1996 Temperature constraints on overwinter survival ofage-0 white perch Transactions of the American Fisheries Society 125 466ndash471

Justic D Rabalais NN Turner RE 2003 Simulated responses of the Gulf ofMexico hypoxia to variations in climate and anthropogenic nutrient loadingJournal of Marine Systems 42 115ndash126

Kana TM Lomas MW MacIntyre HL Cornwell JC Gobler CJ 2004 Stimu-lation of the brown tide organism Aureococcus anophagefferens by selectivenutrient additions to in situ mesocosms Harmful Algae 3 377ndash388

Kaushal SS Likens GE Jaworski NA Pace ML Sides AM Seekell D Belt KTSecor DH Wingate R Rising stream and river temperatures in the UnitedStates Frontiers in Ecology and the Environment in press

Kearney MS Stevenson JC Ward LG 1994 Spatial and temporal changes inmarsh vertical accretion rates at Monie Bay implications for sea-level riseJournal of Coastal Research 10 1010ndash1020

Kemp WM Batiuk R Bartleson R Bergstrom P Carter V Gallegos CHunley W Karrh L Koch E Landwehr J Moore K Murray L Naylor MRybicki N Stevenson JC Wilcox D 2004 Habitat requirements forsubmerged aquatic vegetation in Chesapeake Bay water quality light regimeand physical-chemical factors Estuaries 27 363ndash377

Kemp WM Boynton WR Adolf JE Boesch DF Boicourt WC Brush GCornwell JC Fisher TR Glibert PM Hagy JD 2005 Eutrophication ofChesapeake Bay historical trends and ecological interactions Marine EcologyProgress Series 303 1ndash29

Kemp WM Goldman EB 2008 Thresholds in the Recovery of Eutrophic CoastalEcosystems Chesapeake Bay Program and Maryland Sea Grant 46 pp

Kennedy VS Mihursky JA 1971 Upper temperature tolerances of some estuarinebivalves Chesapeake Science 12 193ndash204

Kennedy VS Twilley RR Kleypas JA Cowan Jr JH Hare SR 2002 Coastal andMarine Ecosystems and Global Climate Change Potential Effects on USResources Pew Center on Global Climate Change Arlington VA 52 pp

Kimmel DG Miller WD Roman MR 2006 Regional scale climate forcing ofmesozooplankton dynamics in Chesapeake Bay Estuaries and Coasts 29375ndash387

Kimmel DG Newell RIE 2007 The influence of climate variation on easternoyster (Crassostrea virginica) juvenile abundance in Chesapeake Bay Limnologyand Oceanography 52 959ndash965

Kneib RT Wagner SL 1994 Nekton use of vegetated marsh habitats at differentstages of tidal inundation Marine Ecology Progress Series 106 227ndash238

Lambert SJ Fyfe JC 2006 Changes in winter cyclone frequencies and strengthssimulated in enhanced greenhouse warming experiments results from themodels participating in the IPCC diagnostic exercise Climate Dynamics 26713ndash728

Langland M Cronin T Phillips S 2003 Executive summary In Langland MCronin T (Eds) A Summary Report of Sediment Processes in Chesapeake Bayand its Watershed United States Geological Survey New Cumberland Penn-sylvania pp 1ndash20

Langland MJ Raffensperger JP Moyer DL Landwehr JM Schwarz GE 2006Changes in Streamflow and Water Quality in Selected Nontidal Basins in theChesapeake Bay Watershed 1985-2004 US Geological Survey ScientificInvestigations Report 2006-5178 75 pp

Langley JA McKee KL Cahoon DR Cherry KA Megonigal JP 2009 ElevatedCO2 stimulates marsh elevation gain counterbalancing sea-level riseProceedings of the National Academy of Sciences of the United States ofAmerica 106 6182ndash6186

Lomas MW Glibert PM Shiah FK Smith EM 2002 Microbial processes andtemperature in Chesapeake Bay current relationships and potential impacts ofregional warming Global Change Biology 8 51ndash70

Lotze HK Worm B 2002 Complex interactions of climatic and ecological controlson macroalgal recruitment Limnology and Oceanography 47 1734ndash1741

Lovett GM Weathers KC Arthur MA 2002 Control of nitrogen loss fromforested watersheds by soil carbon nitrogen ratio and tree species compositionEcosystems 5 712ndash718

Lowrance R Altier LS Newbold JD Schnabel RR Groffman PM Denver JMCorrell DL Gilliam JW Robinson JL Brinsfield RB Staver KW Lucas WTodd AH 1997 Water quality functions of riparian forest buffers in Ches-apeake Bay watersheds Environmental Management 21 687ndash712

Malone TC 1992 Effects of water column processes on dissolved oxygen nutri-ents phytoplankton and zooplankton In Smith DE Leffler M Mackiernan G(Eds) Oxygen Dynamics in the Chesapeake Bay A Synthesis of RecentResearch Maryland Sea Grant College College Park MD pp 61ndash112

Malone TC Conley DJ Fisher TR Glibert PM Harding LW Sellner KG 1996Scales of nutrient-limited phytoplankton productivity in Chesapeake BayEstuaries 19 371ndash385

Malone TC Kemp WM Ducklow HW Boynton WR Tuttle JH Jonas RB1986 Lateral variation in the production and fate of phytoplankton in a partiallystratified estuary Marine Ecology Progress Series 32 149ndash160

Manning MR 2006 The treatment of uncertainties in the Fourth IPCC AssessmentReport Advances in Climate Change Research 2 (Suppl 1) 13ndash21

Marks M Lapin B Randall J 1994 Phragmites australis (P communis) threatsmanagement and monitoring Natural Areas Journal 14 285ndash294

Marshall HG Egerton TA 2009 The increasing occurrence and extendeddevelopment of potentially harmful algal blooms in Virginia tidal rivers

Proceedings of the 2009 Virginia Water Research Conference Water Resourcesin Changing Climates Richmond VA

Marshall HG Egerton T Stem T Hicks J Kokocinski M 2004 Extended bloomconcentrations of Dinophysis acuminata in Virginia estuaries during late winterthrough early spring 2002 In Steidinger KA Landsberg JH Tomas CRVargo GA (Eds) Harmful Algae 2002 Florida Fish amp Wildlife ConservationCommission Florida Institute of Oceanography IOC of UNESCO St PetersburgFL pp 364ndash366

Marshall HG Nesius KK 1996 Phytoplankton composition in relation to primaryproduction in Chesapeake Bay Marine Biology 125 611ndash617

McLaughlin JB DePaola A Bopp CA Martinek KA Napolilli NP Allison CGMurray SL Thompson EC Bird MM Middaugh JP 2005 Outbreak of vibrioparahaemolyticus vastroenteritis associated with Alaskan oysters New EnglandJournal of Medicine 353 1463ndash1470

McMichael AJ Woodruff RE Hales S 2006 Climate change and human healthpresent and future risks The Lancet 367 859ndash869

Meade RH 1988 Movement and storage of sediment in river systems InLerman A Meybeck M (Eds) Physical and Chemical Weathering inGeochemical Cycles Kluwer Dordrecht pp 165ndash179

Meehl GA Stocker TF Collins WD Friedlingstein P Gaye AT Gregory JMKitoh A Knutti R Murphy JM Noda A Raper SCB Watterson IGWeaver AJ Zhao ZC 2007 Global climate projections In Solomon S Qin DManning M Chen Z Marquis M Averyt KB Tignor M Miller HL (Eds)Climate Change 2007 The Physical Science Basis Contribution of WorkingGroup I to the Fourth Assessment Report of the Intergovernmental Panel onClimate Change Cambridge University Press Cambridge United Kingdom andNew York NY USA pp 747ndash845

Meyerson LA Vogt KA Chambers RM 2002 Linking the success of Phragmitesto the alteration of ecosystem nutrient cycles In Weinstein MP Kreeger DA(Eds) Concepts and Controversies in Tidal Marsh Ecology Part 10 SuccessCriteria for Tidal Marsh Restoration Kluwer Academic Publishers New York pp827ndash844

Miller AW Reynolds AC Sobrino C Riedel GF 2009 Shellfish face uncertainfuture in high CO2 world Influence of acidification on oyster larvae calcifica-tion PLoS ONE 4(5) e5661 doi1013761journalpone00056661

Montane MM Austin HM 2005 Effects of hurricanes on Atlantic croaker(Micropogonias undulatus) recruitment to Chesapeake Bay In Sellner KG (Ed)Hurricane Isabel in Perspective Chesapeake Research Consortium EdgewaterMaryland pp 185ndash192

Monteleone DM Duguay LE 2003 Laboratory studies of predation by thectenophore Mnemiopsis leidyi on the early stages in the life history of the bayanchovy Anchoa mitchilli Journal of Plankton Research 10 359ndash372

Moore KA Jarvis JC 2008 Environmental factors affecting recent summertimeeelgrass diebacks in the lower Chesapeake Bay implications for long-termpersistence Journal of Coastal Research 55 135ndash147

Moore KA Wetzel RL 2000 Seasonal variations in eelgrass (Zostera marina L)responses to nutrient enrichment and reduced light availability in experimentalecosystems Journal of Experimental Marine Biology and Ecology 244 1ndash28

Moore KA Wilcox DJ Orth RJ 2000 Analysis of the abundance ofsubmersed aquatic vegetation communities in the Chesapeake Bay Estu-aries 23 115ndash127

Moore MV Pace ML Mather JR Murdoch PS Howarth RW Folt CLChen CY Hemond HF Flebbe PA Driscoll CT 1997 Potential effects ofclimate change on freshwater ecosystems of the New EnglandMid-AtlanticCoastal Region Climate Research 14 161ndash173

Moss RH Malone EL Ramachander S Perez MR 2002 Climate ChangeImpacts Maryland Resources at Risk Maryland Energy Administration 96 pp

Mourino-Perez RR Worden AZ Azam F 2003 Growth of Vibrio cholerae O1 inred tide waters off California Applied and Environmental Microbiology 696923ndash6931

Mulholland MR Morse RE Boneillo G Bernhardt PW Filippino KCProcise LA Blanco-Garcia J Marshall HG Egerton TA Hunley WSMoore KA Berry DL Gobler CJ 2009 Understanding causes and impacts ofthe dinoflagellate Cochlodinium polykrikoides blooms in the Chesapeake BayEstuaries and Coasts 32 734ndash747

Murdy EO Birdsong RS Musick JA 1997 Fishes of the Chesapeake BaySmithsonian Institution Press Washington DC 324 pp

Muzzall PM 1999 Nematode parasites of yellow perch Perca flavescens from theLaurentian Great Lakes Journal of the Helminthological Society of Washington66 115ndash122

Najjar RG 1999 The water balance of the Susquehanna River Basin and itsresponse to climate change Journal of Hydrology 219 7ndash19

Najjar RG 2009 Personal communicationNajjar RG Patterson L Graham S 2009 Climate simulations of major estuarine

watersheds in the Mid-Atlantic region of the United States Climatic Change 95139ndash168

Najjar RG Walker HA Anderson PJ Barron EJ Bord RJ Gibson JRKennedy VS Knight CG Megonigal JP OrsquoConnor RE Polsky CDPsuty NP Richards BA Sorenson LG Steele EM Swanson RS 2000 Thepotential impacts of climate change on the mid-Atlantic coastal region ClimateResearch 14 219ndash233

Nakicenovic N Swart R 2000 Special Report on Emissions Scenarios A SpecialReport of Working Group III of the Intergovernmental Panel on Climate ChangeCambridge University Press Cambridge United Kingdom and New York NYUSA 599 pp


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Orth R Williams M Marion S Wilcox D Moore K Kemp WM Batiuk RBergstrom P Carruthers T Rybicki N Dennison W Long term trends insubmersed aquatic vegetation (SAV) in Chesapeake Bay related to water qualityEstuaries and Coasts in review

Orth RJ Moore KA 1983 Chesapeake Bay An unprecedented decline insubmerged aquatic vegetation Science 222 51ndash53

Osman RW Abbe GR 1994 Post-settlement factors affecting oyster recruitmentin the Chesapeake Bay USA In Dyer K Orth R (Eds) Changes in Fluxes inEstuaries Olsen amp Olsen Denmark pp 335ndash340

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Palacios SL Zimmerman RC 2007 Response of eelgrass Zostera marina to CO2enrichment possible impacts of climate change and potential for remediationof coastal habitats Marine Ecology Progress Series 344 1ndash13

Peperzak L 2003 Climate change and harmful algal blooms in the North Sea ActaOecologica 24S 139ndash144

Perry JE Hershner CH 1999 Temporal changes in the vegetation pattern ina tidal freshwater marsh Wetlands 19 90ndash99

Place AR Saito K Deeds JR Robledo JAF Vasta GR 2008 A decade ofresearch on Pfiesteria spp and their toxins unresolved questions and analternative hypothesis In Botana LM (Ed) Seafood and Freshwater ToxinsCRC Press New York pp 717ndash751

Preston BL 2004 Observed winter warming of the Chesapeake Bay estuary(1949ndash2002) Implications for ecosystem management EnvironmentalManagement 34 125ndash139

Puckett BJ Secor DH Ju SJ 2008 Validation and application of lipofuscin-basedage determination for Chesapeake Bay Blue Crabs Callinectes sapidus Trans-actions of the American Fisheries Society 137 1637ndash1649

Purcell JE 1992 Effects of predation by the scyphomedusan Chrysaora quinque-cirrha on zooplankton populations in Chesapeake Bay USA Marine EcologyProgress Series 87 65ndash76

Purcell JE Arai MN 2000 Interactions of pelagic cnidarians and ctenophoreswith fish a review Hydrobiologia 451 27ndash44

Quinn TP Adams DJ 1996 Environmental changes affecting the migratory timingof American Shad and Sockeye Salmon Ecology 77 1151ndash1162

Rahmstorf S 2007 A semi-empirical approach to projecting future sea-level riseScience 315 368ndash370

Rasse DP Peresta G Drake BG 2005 Seventeen years of elevated CO2 exposurein a Chesapeake Bay Wetland Sustained but contrasting responses of plantgrowth and CO2 uptake Global Change Biology 11 369ndash377

Ratti S Giordano M Morse D 2007 CO2-concentrating mechanisms of thepotentially toxic dinoflagellate Protoceratium reticulatum (dinophyceaeGonyaulacales) Journal of Phycology 43 693ndash701

Riebesell U Schulz KG Bellerby RGJ Botros M Fritsche P Meyerhofer MNeill C Nondal G Oschlies A Wohlers J 2007 Enhanced biological carbonconsumption in a high CO2 ocean Nature 450 545ndash548

Rogers CE McCarty JP 2000 Climate change and ecosystems of the Mid-AtlanticRegion Climate Research 14 235ndash244

Roman MR Gauzens AL Rhinehart WK White JR 1993 Effects of low oxygenwaters on Chesapeake Bay zooplankton Limnology and Oceanography 381603ndash1614

Rombough PJ 1997 The effects of temperature on embryonic and larval devel-opment In Wood CM McDonald DG (Eds) Global Warming Implicationsfor Freshwater and Marine Fish Cambridge University Press New York NY pp177ndash224

Rome MS Young-Williams AC Davis GR Hines AH 2005 Linking tempera-ture and salinity tolerance to winter mortality of Chesapeake Bay blue crabs(Callinectes sapidus) Journal of Experimental Marine Biology and Ecology 319129ndash145

Rose JB Daeschner S Easterling DR Curriero FC Lele S Patz JA 2000Climate and waterborne disease outbreaks Journal American Water WorksAssociation 92 77ndash87

Rost B Richter K Riebesell U Hansen P 2006 Inorganic carbon acquisition inred tide dinoflagellates Plant Cell amp Environment 29 810ndash822

Sagasti A Schaffner LC Duffy JE 2001 Effects of periodic hypoxia on mortalityfeeding and predation in an estuarine epifaunal community Journal of Exper-imental Marine Biology and Ecology 258 257ndash283

Santamarıa L van Vierssen W 1997 Photosynthetic temperature responses offresh-and brackish-water macrophytes a review Aquatic Botany 58 135ndash150

Scavia D Field JC Boesch DF Buddemeier RW Burkett V Cayan DRFogarty M Harwell MA Howarth RW Mason C Reed DJ Royer TCSallenger AH Titus JG 2002 Climate change impacts on US coastal andmarine ecosystems Estuaries 25 149ndash164

Schaefer SC Alber M 2007 Temperature controls a latitudinal gradient in theproportion of watershed nitrogen exported to coastal ecosystems Biogeo-chemistry 85 333ndash346

Schubel JR Pritchard DW 1986 Responses of Upper Chesapeake Bay to varia-tions in discharge of the Susquehanna River Estuaries 9 236ndash249

Secor DH Gunderson TE 1998 Effects of hypoxia and temperature on survivalgrowth and respiration of juvenile Atlantic sturgeon Acipenser oxyrinchusFishery Bulletin 96 603ndash613

Sellner KG 1987 Phytoplankton in Chesapeake Bay role in carbon oxygen andnutrient dynamics In Majumdar SK Hall Jr LW Austin HM (Eds)Contaminant Problems and Management of Living Chesapeake Bay ResourcesPennsylvania Academy of Sciences Philadelphia PA pp 134ndash157

Sellner KG Olson MM 1985 Copepod grazing in red tides of Chesapeake Bay InAnderson DM White AW Baden DG (Eds) Toxic Dinoflagellates ElsevierNew York pp 245ndash251

Sellner KG Sawangwong P Dawson R Boynton WR Kemp WM Garber JH1992 Fate of dinoflagellates in Chesapeake Bay Is sedimentation likely InSmayda TJ (Ed) Marine Phytoplankton Elsevier New York NY pp 825ndash830

Sharpley AN Hedley MJ Sibbesen E Hillbricht-Ilkowska A House WARyszkowski L 1995 Phosphorus transfers from terrestrial to aquatic ecosys-tems In Tiessen H (Ed) Phosphorus in the Global Environment John WileyChichester UK pp 173ndash242

Shimps EL Rice JA Osborne JA 2005 Hypoxia tolerance in two juvenileestuary-dependent fishes Journal of Experimental Marine Biology and Ecology325 146ndash162

Short FT Neckles HA 1999 The effects of global climate change on seagrassesAquatic Botany 63 169ndash196

Silvert W 1993 Size-structured models of continental shelf food webs InChristensen V Pauly D (Eds) Trophic Models of Aquatic Ecosystems ICLARMConference Proceedings No 26 ICLARM Manila Philippines pp 40ndash43

Smith EM Kemp WM 1995 Seasonal and regional variations in planktoncommunity production and respiration for Chesapeake Bay Marine EcologyProgress Series 116 217ndash231

Smith S Herman J Cronin T Schwarz G Langland M Patison K Linker L2003 Chapter 7 Integrated approaches to sediment studies In Langland MCronin T (Eds) A Summary Report of Sediment Processes in Chesapeake Bayand its Watershed United States Geological Survey New Cumberland Penn-sylvania pp 80ndash98

Stachowicz JJ Terwin JR Whitlatch RB Osman RW 2002 Linking climatechange and biological invasions Ocean warming facilitates nonindigenousspecies invasions Proceedings of the National Academy of Sciences of theUnited States of America 99 15497ndash15500

Stevenson JC Staver LW Staver KW 1993 Water quality associated withsurvival of submersed aquatic vegetation along an estuarine gradient Estuaries16 346ndash361

Sullivan BK Van Keuren D Clancy M 2001 Timing and size of blooms of thectenophore Mnemiopsis leidyi in relation to temperature in Narragansett BayRI Hydrobiologia 451 113ndash120

Swaney DP Sherman D Howarth RW 1996 Modeling water sediment andorganic carbon discharges in the HudsonndashMohawk basin coupling to terrestrialsources Estuaries 19 833ndash847

Tebaldi C Smith RL Nychka D Mearns LO 2005 Quantifying uncertainty inprojections of regional climate change a Bayesian approach to the analysis ofmultimodel ensembles Journal of Climate 18 1524ndash1540

Teng H Washington WM Meehl GA 2007 Interannual variations and futurechange of wintertime extratropical cyclone activity over North America inCCSM3 Climate Dynamics 1ndash14 doi101007s00382-007-0314-1

Thom RM 1996 CO2-Enrichment effects on eelgrass (Zostera marina L) and bullkelp (Nereocystis luetkeana (mert) P amp R) Water Air amp Soil Pollution 88383ndash391

Titus JG Richman C 2001 Maps of lands vulnerable to sea level rise modeledelevations along the US Atlantic and Gulf coasts Climate Research 18205ndash228

Tortell PD DiTullio GR Sigman DM Morel FMM 2002 CO2 effects on speciescomposition and nutrient utilization in an Equatorial Pacific phytoplanktonassemblage Marine Ecology Progress Series 236 37ndash43

Trenberth KE Jones PD Ambenje P Bojariu R Easterling D Klein Tank AParker D Rahimzadeh F Renwick JA Rusticucci M Soden B Zhai P 2007Observations Surface and atmospheric climate change In Solomon S Qin DManning M Chen Z Marquis M Averyt KB Tignor M Miller HL (Eds)Climate Change 2007 The Physical Science Basis Contribution of WorkingGroup I to the Fourth Assessment Report of the Intergovernmental Panel onClimate Change Cambridge University Press Cambridge United Kingdom andNew York NY USA pp 235ndash336


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US Environmental Protection Agency 2008a Effects of Climate Change on AquaticInvasive Species and Implications for Managementand Research EPA600R-08014 US Environmental Protection Agency Washington DC 337 pp

US Environmental Protection Agency 2008b A Screening Assessment of thePotential Impacts of Climate Change on Combined Sewer Overflow (CSO)Mitigation in the Great Lakes and New England Regions (Final Report)EPA600R-07033FUS Environmental Protection Agency WashingtonDC 52 pp

US Environmental Protection Agency 2008c A Screening Assessment of thePotential Impacts of Climate Change on the Costs of Implementing WaterQuality-Based Effluent Limits at Publicly-Owned Treatment Works (POTWs) inthe Great Lakes Region (External Review Draft) US Environmental ProtectionAgency EPA600R-07034AUS Environmental Protection Agency Wash-ington DC 35 pp

UNEPGPA 2006 The State of the Marine Environment Trends and ProcessesUNEPGPA The Hague 44 pp

Venterea RT Lovett GM Groffman PM Schwarz PA 2003 Landscape patternsof net nitrification in a northern hardwood-conifer forest Soil Science Societyof America Journal 67 527ndash539

Wardrop DH Kentula ME Jensen SF Stevens Jr DL Hychka KCBrooks RP 2007a Assessment of wetlands in the Upper Juniata Watershedin Pennsylvania USA using the hydrogeomorphic approach Wetlands 27432ndash445

Wardrop DH Kentula ME Stevens Jr DL Jensen SF Brooks RP 2007bRegional assessments of wetland condition An example from the Upper Juniatawatershed in Pennsylvania USA Wetlands 27 416ndash431

Weisberg SB Morin RP Ross EA Hirshfield MF 1986 Eustrongylides (Nem-atoda) infection in mummichogs and other fishes of the Chesapeake Bay regionTransactions of the American Fisheries Society 115 776ndash783

Winter TC 2000 The vulnerability of wetlands to climate change A hydrologic land-scape perspective Journal of the American Water Resources Association 36 305ndash312

Wolock DM McCabe GJ 1999 Estimates of runoff using water-balance andatmospheric general circulation models Journal of the American WaterResources Association 35 1341ndash1350

Wood RJ 2000 Synoptic scale climatic forcing of multispecies fish recruitmentpatterns in Chesapeake Bay PhD thesis Virginia Institute of Marine ScienceThe College of William and Mary

Wood RJ Austin HM 2009 Synchronous multidecadal fish recruitment patternsin Chesapeake Bay USA Canadian Journal of Fisheries and Aquatic Sciences 66496ndash508

Wood RJ Boesch DF Kennedy VS 2002 Future consequences of climate changefor the Chesapeake Bay ecosystem and its fisheries American Fisheries SocietySymposium 32 171ndash184

Wu SY Najjar RG Siewert J 2009 Potential impacts of sea-level rise on the Mid-and Upper-Atlantic Region of the United States Climatic Change 95 121ndash138

Zervas C 2001 Sea level variations of the United States 1854ndash1999 NOAA Tech-nical Report NOS CO-OPS 36 National Ocean Service Silver Spring MD 66 pp

Zhong L Li M Foreman MGG 2008 Resonance and sea level variability inChesapeake Bay Continental Shelf Research 28 2565ndash2573

76degW80degW

46degN

42degN

38degN

Chesapeake Bay watershed

Chesapeake Bay

VA

WV

PA

NY

MDDE

NJ

Atlantic O

cean

0 250 500125km

Fig 1 Map of the Chesapeake Bay its watershed and the states in its watershed


climate change due to human activity well underway (IPCC 2007)there is a need to assess the impacts of climate change on estuaries

The goal of this paper is to review and synthesize the scientificliterature on climate change impacts on the Chesapeake Bay (Fig 1)one of the largest and most productive estuaries in the world(National Oceanic and Atmospheric Administration 1990) In 2000commercial fisheries landings for the Chesapeake Bay exceeded US$172 million accounting for 5 of the value of all United Statesfisheries (National Marine Fisheries Service 2001) Although thesefigures are significant they understate the value of the ChesapeakeBay and its fisheries because they do not account for the ecologicaland recreational services the Bay provides to the food web andfisheries of the North American Atlantic Coast

In this review we follow a logical progression from changes inclimatic and hydrological forcing factors (Section 2) to changes inwatershed fluxes of nutrients and sediment (Section 3) and physical(Section 4) and biogeochemical (Section 5) conditions in the BayImpacts on Bay living resourcesdvascular plants (Section 6) and fishand shellfish (Section 7)dare then considered before summarizingour findings and providing general recommendations for furtherstudy (Section 8) Our synthesis builds on a number of reviews dis-cussing the impact of climate change on ecosystems coastal areasand marine resources of the mid-Atlantic region (Boesch 2008Moore et al 1997 Moss et al 2002 Najjar et al 2000 Rogers andMcCarty 2000 Wood et al 2002) the United States (Field et al2001 Scavia et al 2002) and the World (Kennedy et al 2002)

Statistically rigorous forecast probabilities are not possible atthis time for climate change and its impacts in the Chesapeake Bayregion By using expert judgment however we believe that we candefensibly assign rough probabilities regarding the direction ofmany changes To maintain consistency throughout this paper weuse the following terminology (Manning 2006) to express proba-bility ranges virtually certain (gt99) very likely (90ndash99) andlikely (66ndash90)

2 Climatic and hydrologic processes affecting the bay

21 Atmospheric composition

Being a well-mixed gas in the atmosphere regional and globalprojections of atmospheric CO2 are essentially identical Projectionsfor global mean atmospheric CO2 concentration over the next 100years vary widely mainly because of the uncertainty in future CO2

emissions (Fig 2) but also because of poorly understood feedbacksbetween climate and the carbon cycle However it is virtuallycertain that CO2 levels will continue to increase throughout the 21st

century Surface water CO2 changes are expected to closely trackatmospheric CO2 changes leading to a decrease in pH andcarbonate ion concentration [CO3

2] which is expected to harmCaCO3-secreting organisms including shellfish Orr et al (2005)showed that [CO3

2] and pH decreases averaging about 10 and 01respectively have already taken place throughout the surface oceandue to the invasion of anthropogenic CO2 Under a greenhouse gasscenario similar to the Intergovernmental Panel on ClimateChangersquos (IPCCrsquos) A2 storyline (Fig 2) these changes increase to45 and 05 respectively by 2100

22 Water temperature

Fig 3 shows 20th-century surface water temperature variabilitymeasured at two locations in the Chesapeake Bay High variability issuperimposed on a long-term warming the 1990s were about 1 Cwarmer than the 1960s Fig 3 also shows an estimate of surfacewater temperature averaged over the mainstem Bay based on datafrom the Chesapeake Bay Water Quality Monitoring Program

which sampled the water column at least monthly at several dozenstations throughout the mainstem Bay since 1984 The correspon-dence between the pier data and the Bay-average data during theperiod of overlap indicates that the longer time series measured atthe piers reflect mean Bay temperature quite well

Numerous studies have documented a positive correlationbetween water temperature in the Bay and regional atmosphericand oceanic temperature at time scales ranging from monthly todecadal (eg Cronin et al 2003 Preston 2004) It is therefore verylikely that the regional temperature projections made from climatemodels can be accurately applied to the Bay

Two recent studies have analyzed the output of global climatemodels (GCMs) in the Chesapeake Bay region (Hayhoe et al 2007Najjar et al 2009) Both studies found the multi-model average tocapture the 20th-century warming trend of the northern portion ofthe Chesapeake Bay watershed but the weak cooling observed inits southern portion (eg Allard and Keim 2007) is not found inmodels (Najjar et al 2009) The degree of projected warmingdiffers greatly among models (Fig 4) and scenario (Fig 5) but it isnevertheless very likely that temperature will continue to increasethroughout the 21st century Model-averaged projections in Najjaret al (2009) for the six scenarios shown in Fig 2 range from 3 to6 C warming by 2070ndash2099 (Fig 5a) When the best-performingmodels are used the projected warming decreases to 2ndash5 C(Fig 5c) Similar projections were found by Hayhoe et al (2007)

Changes in temperature extremes can be as important as annualmean temperature changes Meehl et al (2007) analyzed theoutput of nine global climate models for changes in heat wavesdefined as lsquolsquothe longest period in the year of at least five consecutivedays with maximum temperature at least 5 C higher than theclimatology of the same calendar dayrsquorsquo Under the A1B scenario(Fig 2) heat waves along the east coast of North America includingthe Mid-Atlantic are projected to increase by more than twostandard deviations by the end of the 21st century

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potential climate-change impacts on the chesapeake bay

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