Exploring causes of increasing Exploring causes of increasing nitrate concentrations in the nitrate concentrations in the

Lower St. Croix RiverLower St. Croix River

Brenda Moraska Lafrancois, Byron Karns, Brenda Moraska Lafrancois, Byron Karns, Wade Miller, Brian Schuetz, Elizabeth Wade Miller, Brian Schuetz, Elizabeth

Peterson, and Will DanielsPeterson, and Will Daniels

K. Heideman

Why N?Why N?

•• Like P, N can be a Like P, N can be a limiting nutrientlimiting nutrient

•• Excess N (or changes Excess N (or changes in N:P) can lead to in N:P) can lead to nuisance bloomsnuisance blooms

Coastal Dead ZonesCoastal Dead Zones• Spread exponentially since 1960s• 400 ecosystems• Result of coastal eutrophication• Fertilizer runoff • Serious ecological consequences

Diaz and Rosenberg. 2008. Science 321:926-929.

Gulf of Mexico HypoxiaGulf of Mexico Hypoxia•• Noticed since 1950s, Noticed since 1950s,

monitored since 1970smonitored since 1970s•• Driven by increases in Driven by increases in

nitrate (NOnitrate (NO33--N) N) –– fertilizer fertilizer •• 56% of nitrate from Upper 56% of nitrate from Upper

MississippiMississippi•• 30% reduction in N 30% reduction in N

loading requiredloading required

Goolsby (2000) Eos. Rabalais et al. (2002) BioScience.

Lake St. Croix NitrateLake St. Croix Nitrate

D. Morrison, MPCA

0.645 mg/l

1.590 mg/l

Median values for SC 0.3 and MR 796.9, 2000-2004.

Lake St. Croix NitrateLake St. Croix Nitrate

1.0

0.8

0.6

0.4

0.2

0.0

19751980

19851990

19952000

Seasonal Kendall TestPrescott (1976-2002)p < 0.05

Nitr

ate-

N (m

g/l) Median (1990-1999)

0

0.2

0.4

0.6

EPA Reference

(Rivers)Prescott

Stillwater

Nitr

ate-

N (m

g/l)

Potential CausesPotential Causes•• Increases in fertilizer useIncreases in fertilizer use•• Changes in wastewater Changes in wastewater

treatment (NHtreatment (NH33→→NONO33))•• Atmospheric depositionAtmospheric deposition•• Changes in soil Changes in soil

denitrification processesdenitrification processes

Stable Isotope AnalysisStable Isotope Analysis

•• What are stable isotopes?What are stable isotopes?–– Different forms of the same element Different forms of the same element –– Not radioactiveNot radioactive–– Common ones = C, N, O, S, HCommon ones = C, N, O, S, H

•• Relative proportions used to infer things about Relative proportions used to infer things about ecosystem structure and processecosystem structure and process

•• Isotopes of N and O useful for tracking nitrateIsotopes of N and O useful for tracking nitrate–– 1414NN and and 1515NN–– 1616OO, , 1717O, and O, and 1818OO

Nitrate Stable IsotopesNitrate Stable Isotopes

C. Kendall. 1998. Tracing nitrogen sources and cycling in catchmC. Kendall. 1998. Tracing nitrogen sources and cycling in catchments. Chapter 16 ents. Chapter 16 inin C. Kendall C. Kendall and J.J. McDonnell, eds., Isotope tracers in catchment hydrologyand J.J. McDonnell, eds., Isotope tracers in catchment hydrology. Elsevier Science B.V.. Elsevier Science B.V.

Study GoalsStudy Goals

1.1. Characterize patterns in nitrate Characterize patterns in nitrate concentration concentration •• Mainstem Lake St. CroixMainstem Lake St. Croix•• TributariesTributaries•• Wastewater Treatment PlantsWastewater Treatment Plants

2.2. Identify nitrate sourcesIdentify nitrate sources3.3. Evaluate usefulness of biological proxiesEvaluate usefulness of biological proxies

HypothesesHypotheses

•• NONO33--N concentrations are highest in N concentrations are highest in downstream tributaries and WWTPsdownstream tributaries and WWTPs

•• δδ1515N and N and δδ1818O suggest wastewater O suggest wastewater influence at upstream sites, fertilizer influence at upstream sites, fertilizer downstreamdownstream

•• δδ1515N is more stable in zebra mussels than N is more stable in zebra mussels than in water or phytoplankton over seasonin water or phytoplankton over season

DesignDesign•• 14 sites14 sites

–– 7 mainstem7 mainstem–– 5 tributaries5 tributaries–– 2 wastewater plants2 wastewater plants

•• 5 sampling events5 sampling events•• 3 components3 components

–– Water (nitrate)Water (nitrate)–– PhytoplanktonPhytoplankton–– Zebra musselsZebra mussels

Stillwater

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

Hudson

Prescott

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

KinnickinnicRiver

Hudson

Prescott

2

3

4

5

6

7

1Brown’s Creek

Stillwater

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

Hudson

Prescott

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

KinnickinnicRiver

Hudson

Prescott

2

3

4

5

6

7

1Brown’s Creek

Stillwater

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

Hudson

Prescott

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

KinnickinnicRiver

Hudson

Prescott

2

3

4

5

6

7

1Brown’s Creek

Stillwater

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

Hudson

Prescott

Bayport

Afton

Mississippi River

Valley Creek

Trout Brook

Willow River

KinnickinnicRiver

Hudson

Prescott

2

3

4

5

6

7

1Brown’s Creek

MethodsMethods

Phytoplankton

Water – NO3-N Zebra Mussels

MethodsMethods

•• Water analyzed for Water analyzed for nitrate concentration nitrate concentration and stable isotopes of N and stable isotopes of N and Oand O

–– Diffusion technique (Diffusion technique (δδ1515N) N) –– Denitrifier method (Denitrifier method (δδ1515N N

and and δδ1818O)O)

•• Phytoplankton and Phytoplankton and zebra mussel tissue zebra mussel tissue analyzed for analyzed for δδ1515N (N (δδ1313C) C)

Mainstem ConcentrationsMainstem Concentrations

Means and StDevs, May-Aug 2008Note: detection limit = 0.2 mg/l

0.00

0.10

0.20

0.30

0.40

0.50

SC1 SC2 SC3 SC4 SC5 SC6 SC7

Nitr

ate-

N (m

g/l)

High Bridge Prescott

Tribs and WWTPsTribs and WWTPs

Means and StDevs, May-Aug 2008

0

4

8

12

16

Browns

MallalieuTrout

KinnickValley

Hudson WWTP

St. Croix Valley WWTP

Nitr

ate-

N (m

g/l)

Stable Isotope ResultsStable Isotope Results(Hypothetical)(Hypothetical)

Measured Measured St. Croix sitesSt. Croix sites

δδ1515NN W

aste

Was

te

Ferti

lizer

Ferti

lizer

ExpectedExpectedSource SignalsSource Signals

Preliminary FindingsPreliminary Findings

•• Nitrate increases Nitrate increases systematicallysystematically–– Largest increases Largest increases

downstreamdownstream–– Lake St. Croix Lake St. Croix

tributaries and tributaries and tributary land use tributary land use importantimportant

AcknowledgementsAcknowledgements•• U.S. Army Corps of U.S. Army Corps of

EngineersEngineers•• National Park ServiceNational Park Service•• SCWRSSCWRS•• Metropolitan CouncilMetropolitan Council•• Colorado Plateau Stable Colorado Plateau Stable

Isotope LabIsotope Lab•• Dennis Lindeke, Jim Dennis Lindeke, Jim

SchreiberSchreiber•• David VanderMeulen David VanderMeulen •• Toben Lafrancois and the Toben Lafrancois and the

Youth River Action Youth River Action ProgramProgram

Seasonal PatternsSeasonal Patterns

Long-term means (1976-2002)

Jan FebMar

AprMay

Jun JulAug

Sep Oct NovDec

0

0.3

0.6

0.9

1.2

1.5

Nitr

ate-

N (

mg/

l)

HydrographStillwaterPrescott

0

4000

8000

12000

16000

20000

Dis

char

ge (c

fs)

Seasonal PatternsSeasonal Patterns

Means and StDevs, May-Aug 2008

0.00

0.10

0.20

0.30

0.40

May June July Aug

Nitr

ate-

N (m

g/l)

Mainstem St. Croix Sites