Nutrient and salinity controls on Phragmites australis invasion in

Cape Cod salt marsh ecosystems

Leena L. Vilonen

Abstract

Phragmites australis is a highly invasive salt marsh grass. Invasive species pose a

serious threat to native plant species and the biodiversity of ecosystems. In my study, I

looked into the effects of salinity and nitrogen on P. australis distribution, as well as, the

efficiency of nitrogen uptake by P. australis, the percent cover of P. australis, and

species richness in P. australis segments. To obtain these measurements I first identified

P. australis segments and measured the shoreline length with P. australis and shoreline

length without P. australis. I then measured pore water salinity, ground water inorganic

nitrogen, carbon and nitrogen contents of leaves, percent cover in quadrats, and number

of species in each segment. I found that P. australis covered more shoreline in low

salinity and low nitrogen areas. I also found lower carbon to nitrogen ratios in P.

australis than in the other salt marsh species. I found that salinity and nitrogen had no

effect on P. australis cover and species richness decreased by 24% in P. australis

segments. In conclusion, P. australis thrived best in low salinity and low nitrogen

environments, but P. australis did not seem to be affected by nitrogen concentrations. P.

australis decreased species richness, but not as dramatically as I expected.

Keywords

Phragmites australis, Salinity, Nitrogen, Species Cover, Species Richness

Introduction

Invasive species pose a major threat to native plant species and biodiversity of

ecosystems. The Office of Technology Assessment has estimated that the United States

has over 4,500 invasive species. It has been estimated that fifty-seven percent of

imperiled plants species are affected by invasive plant species (Wilcove et al. pg. 242).

Phragmites australis is a highly invasive plant species found in North American

salt marshes. P. australis can thrive in variable conditions ranging from freshwater

systems to high salinity salt marshes. No evidence as of yet has been found that the

abundance of P. australis differs in any of these systems (Chambers et al. pg. 263).

Anthropogenic disturbances of salt marshes through the addition of excess freshwater and

nitrogen into the system has furthered aided in the invasion of P. australis (Bertness 2003

pg. 1404). It has been shown that manipulating water tables to control salinity can

mitigate P. australis invasion (Hellings et al. pg. 48).

In a typical salt marsh, P. australis can drive down native species abundance by

as much as ninety-four percent (Bertness 2004 pg. 1430). This change in species

composition has harmful effects on invaded salt marshes. Effects of this invasion include

the depletion of food sources for wildlife inhabiting tidal marshes, an increase in

flammability of the system due to the increase in dry P. australis tinder, and a rise in the

elevation of the salt marsh causing a decrease in saltwater flooding (U.S. Fish and

Wildlife Service 2007).

In this study, I am examining the variables that allow P. australis to outcompete

native salt marsh species. The main question driving my research is how P. australis

strongly outcompetes native salt marsh species. Five further questions stemming from my

main question guide my experimental design. The first question is whether salinity has an

effect on P. australis distribution. The second question is if nitrogen availability has an

effect on P. australis distribution. The third question is if P. australis or native salt marsh

plants take up nitrogen more efficiently. The fourth question is if salinity or nitrogen

affect the percent cover or P. australis or native salt marsh plants. The fifth question is if

P. australis has an effect on species richness in the ecosystem. Through these five

questions, I will hopefully be able to draw a conclusion on how P. australis is such a

potent invasive species.

Methods

To obtain a more complete understanding, I choose six sites all around Cape Cod

Massachusetts that varied in salinity and nitrogen (figure 1). The first site I obtained data

from was East Sandwich Game Farm in Sandwich Massachusetts. A railroad intersects

this site, which cut off the saltwater input to part of the salt marsh. One part of the salt

marsh had lost the saltwater input into the salt marsh, but was restored in 2009. The last

area never lost saltwater inputs. A forest preserve surrounds East Sandwich Game Farm

and no houses surround the area. The second site is Little Sippewissett situated in

Falmouth, Massachusetts. Little Sippewissett is separated by a beach and parking lot

from the ocean, but still maintains a saltwater input. Houses surround the Little

Sippewisset marsh entirely. The third site, West Falmouth Harbor located in Falmouth

Massachusetts, feeds directly into the ocean. Houses directly surround the estuary. The

West Falmouth Waste Treatment Facility also feeds into the harbor. The fourth site,

Oyster Pond located in Falmouth Massachusetts, is separated from the ocean by the

Shining Sea bike path. The area around Oyster Pond is fairly populated. A highway

preventing saltwater inflow intersects the fifth site, a salt marsh located in West

Barnstable Massachusetts. The area around the salt marsh is mostly uninhabited. The

sixth site, Sage Lot Pond in Mashpee Massachusetts, directly feeds into the ocean. The

area around the estuary is mostly uninhabited.

To test my hypothesis, I took shoreline measurements across these six different

sites. I used segments of P. australis and segments of native salt marsh species in similar

salinity zones for comparison. I tested five different variables: pore water salinity,

nitrogen available to the salt marsh, percent cover of the plants present, species richness,

and leaf chemistry of the plant species present. I also measured the distance along the

shore of P. australis segments and non-P. australis segments using google earth. In each

type of segment, I took all five measurements. These segments first fell into two different

salinity categories: high salinity and low salinity. Any salinity under eighteen parts per

thousand (ppt) I considered low salinity and any salinity eighteen and above ppt I

considered high salinity. These segments were also divided into high and low nitrogen

categories. Any nitrogen in the ground water under fourteen micromoles I considered low

nitrogen and any ground water above or equal to fourteen micromoles I considered high

nitrogen.

The first variable I measured was the salinity in segments of tidal marsh. To

measure salinity, I took pore water samples through soil cores. I extracted water from the

soil core and took a salinity measurement using a refractometer. I took one measurement

in each segment.

The second variable I measured is nitrogen available to the salt marsh system. I

took these samples inland of the segments to measure the nitrogen load from outside the

system. I measured this by using a well-point sampler to extract ground water. I stored

my water samples in scintillation vials using a syringe and swinnex filter to filter the

water. I immediately placed the scintillation vials on ice. Once in lab, I placed half the

ground water samples in the freezer for nitrate analysis and added 5 M Hydrochloric Acid

to the other half for ammonium analysis. I put the ground water samples measured for

ammonium in the fridge right after acidification. I then measured the nitrate and

ammonium concentrations for each sample. For ammonium concentrations, I used a

spectrophotometer using the methods outlined in A Practical Handbook of Seawater

Analysis (Strickland 1972). I made a standard curve by diluting known concentrations of

ammonium and then using colorimetric analysis. For nitrate concentrations I used a

latchet machine using the methods outlined in Standard Operating Procedure for Nitrate

(1995).

The third variable I measured was biodiversity. In each P. australis and non-P.

australis segments, I evaluated the number of species in the segment and the overall

coverage of all species present by using a half a meter by half a meter quadrat.

The fourth variable I measured was leaf chemistry in both native salt marsh plants

and P. australis. At my sites, I took leaf samples of P. australis and native salt marsh

plants. In lab, I dried the leaves in a drying oven. Once the leaves dried, I ground the

leaves. To analyze the leaves, I used a NC Soil Analyzer model Flash 2000. To prepare to

use this machine, I packed my 5 to 6 milligrams of the leaf samples in a 9 x 10 millimeter

tin capsule. I packed blank samples that had no contents other than the tin capsules. I also

packed standards with aspartic acid to make a standard curve for my samples.

Results

Through my research, I found that in low salinity zones of all six sites P. australis

covered 81.62% of the overall shoreline. In high salinity zones, P. australis covered

80.91% of the overall shoreline (figure 2). Over the six estuaries, west Barnstable had the

highest shoreline cover of P. australis (97%) and West Falmouth Harbor had the lowest

(13.88%) with Oyster Pond (60.48%), Sage Lot Pond (48%), Little Sippewisset

(47.17%), and East Sandwich Game Farm (45.74%) in between. West Barnstable had the

lowest salinity (1 ppt) and Little Sippewisset had the highest (22.75 ppt) with East

Sandwich Game Farm (13.2 ppt), West Falmouth Harbor (18.25 ppt), and Sage Lot Pond

(20.3 ppt) in between (table 1). I discovered a negative correlation between the average

salinity of the estuaries and the percent of shoreline covered by P. australis with an r-

squared of 0.57 (figure 3).

In low nitrogen zones in all sites P. australis covered 69.59% of the overall

shoreline. In high nitrogen zones, P. australis covered 30.41% of the overall shoreline

(figure 4). In low salinity and low nitrogen zones, P. australis covered 97.10% of the

shoreline. In low salinity and high nitrogen zones, P. australis covered 46.39% of the

shoreline. In high salinity and low nitrogen zones, P. australis covered 15.08% of the

shoreline. In high salinity and high nitrogen zones, P. australis covered 22.66% of the

shoreline (figure 5). Using a weighted average by shoreline length, I found the average

nitrogen concentration at East Sandwich Game Farm to be 18.78 μM. At Little

Sippewissett marsh it was 52.5 μM. At West Falmouth Harbor it was 76.56 μM. At

Oyster Pond it was 20.97 μM. At West Barnstable it was 3.78 μM. At Sage Lot Pond it

was 9.79 μM (table 1). I discovered a negative correlation between the percent shoreline

cover of each estuary and the average nitrogen concentration with an r-squared of 0.6

(figure 6). I also discovered a negative correlation between percent shoreline cover of

each estuary and the average nitrogen concentration of each estuary except in only low

salinity areas with an r-squared of 0.9 (figure 7).

Through my research, I found that the average carbon to nitrogen ratio for P.

australis was similar to that of Spartina alterniflora. Spartina cynosuroides had a much

smaller carbon to nitrogen ratio than P. australis, and Spartina patens and Typha latifolia

had much larger carbon to nitrogen averages than P. australis (figure 8). In low and high

nitrogen zones, the carbon to nitrogen ratio of P. australis stayed the same. The carbon to

nitrogen ratio of spartina species was higher in low nitrogen zones than high nitrogen

zones (figure 9). I found no correlation between the carbon to nitrogen ratio of plants and

nitrogen concentration (figure 10).

I found no correlation between P. australis cover in quadrats over salinity (figure

11). I also found no correlation between S. alterniflora cover in quadrats over salinity

(figure 12). The percent cover of S. patens increased with salinity (figure 13). The

percent cover of T. latifolia decreased with salinity (figure 14). The percent cover of all

spartina species increased with salinity (figure 15). I found no correlation between P.

australis and nitrogen concentration (figure 16) or S. alterniflora and nitrogen

concentration (figure 17). The percent cover of S. patens decreased with nitrogen

concentration (figure 18).

Though my research, I identified four different plant species in non-P. australis

segments. In P. australis segments, I only identified P. australis. The percent cover of P.

australis did not vary significantly (figure 19). The average species richness (number of

species found per segment) in P. australis was 1. In non-P. australis segments the

average species richness was 1.4 (figure 20).

Discussion

Through my findings, salinity seems to be the strongest factor in P. australis

invasion (figure 2). P. australis through both my research and other research done on the

topic has been shown to invade strongly in low salinity areas. Samuel Hellings and John

Gallagher found that at incremental steps of salinity, P. australis abundance decreased

significantly (pg. 44). Brian Silliman and Mark Bertness found that decreased soil salinity

due to shoreline development increased P. australis abundance significantly (pg. 1428).

Randolph Chambers showed that disturbances in the hydrological cycle facilitated P.

australis invasion (pg. 261).

Nitrogen on the other hand seemed to not have as large as an effect on P. australis

invasion. Although I found a larger amount of P. australis in low nitrogen areas (figure

4) and a correlation of average estuary nitrogen concentration and percent estuary

shoreline cover (figure 5), the carbon to nitrogen ratio I found showed a different trend

(figure 8). I expected from my nitrogen results that the carbon to nitrogen ratio would be

lower in higher in P. australis leaves than native salt marsh species; however, the

opposite occurred. These results may have turned out differently if I had measured the

carbon and nitrogen contents of the P. australis stems. Interestingly, the carbon to

nitrogen ratio of P. australis did not change between low and high nitrogen segments, but

the carbon to nitrogen ratio of all spartina species saw a large decrease from low nitrogen

to high nitrogen areas (figure 9). This indicates that the amount of nitrogen in the system

does not have a large effect on P. australis growth, but the amount of nitrogen in the

system does have an effect on spartina species. This trend may be able to explain why P.

australis invades better in low nitrogen segments. Since P. australis seems to not be

affected by the amount of nitrogen available according to the carbon to nitrogen ratios I

found, P. australis could more easily invade lower nitrogen areas, since spartina species

do seem to be affected by the amount of nitrogen available. Todd Minchinton and Mark

Bertness found similar results in their study on P. australis. Minchinton and Bertness

added nutrients to P. australis plots and found that this addition of nutrients had no

significant impact on total P. australis biomass (pg. 1409). Lisa Windham and Laura

Meyerson found that P. australis invasion alters salt marsh nitrogen pools and fluxes (pg.

458). This would explain the trends that both I and Minchinton and Bertness found. The

amount of nutrients available to P. australis have no effect on plant growth if P. australis

itself can alter pools and fluxes to increase the nitrogen available.

Interestingly, I found that the cover of P. australis did not depend on salinity

although the distribution of P. australis depended highly on salinity (figure 11). This

finding indicates that P. australis has the ability to thrive in various levels of salinity.

Windham and Meyerson found a similar trend that P. australis biomass did not vary

between different salinity plots (pg. 454). S. patens, however, cover increased as salinity

increased (figure 13). All spartina species cover increased as salinity increased (figure

15). T. latifolia cover decreased significantly as salinity decreased (figure 14). All plant

species other than P. australis showed a correlation between salinity and cover. This

implies that P. australis outcompetes S. patens and S. alterniflora in low salinity more

than P. australis thrives better in low salinity. I also found that P. australis cover did not

vary with nitrogen concentration (figure 16). S. patens cover on the other hand decreased

significantly as nitrogen concentration increased (figure 18). This again shows that P.

australis does not depend on the concentration of nitrogen, but S. patens does. The

percent cover of P. australis did not vary much, but the percent covers of S. patens, S.

alterniflora, and T. latifolia varied largely (figure 19). This further implies that P.

australis is not very affected by environmental factors and therefore is able to

outcompete the species that are affected.

All P. australis segments only contained P. australis plants (figure 20). Segments

without P. australis contained on average 1.4 different species. P. australis decreased

species biodiversity, but not as significantly as I expected. Bertness and Silliman found a

94% decrease in plant biodiversity (pg. 1430), while I only found a 28 % decrease in

plant biodiversity. I took my measurements very late in the growing season, and therefore

possibly missed various different species. I also likely missed high diversity, low nitrogen

fresh marshes.

Conclusions:

In conclusion, I found that P. australis thrived in low salinity and low nitrogen

areas. I also found that P. australis was less efficient at taking up nitrogen than the native

salt marsh species, which indicated that P. australis is not dependent on nitrogen. I found

that P. australis cover did not depend on salinity or nitrogen, which shows that P.

australis can live in variable environments. Lastly, I found that P. australis decreased

species richness. To prevent the invasion of P. australis, I would suggest manipulating

water tables to increase salt water input and decrease freshwater input.

Acknowledgments:

I would like to thank my mentor Chris Neill for all his help designing my project,

interpreting my data, and writing my report. I would like to thank John Schade for

helping me design my project. I would like to thank Nick Barrett for helping me with my

fieldwork. I would like to thank Rich McHorney, Fiona Jevon, and Tyler Messerschmidt

for helping me with my laboratory work.

Work Cited:

Bertness, Mark D., and Christine Holdredge. "Litter legacy increases the competitive

advantage of invasive P. australis australis in New England wetlands." Conservation

Biology 18.5 (2004): 1424-34.

Bertness, Mark D., and Todd E. Minchinton. "Disturbance-Mediated Competition and

the Spread of P. australis Australis in a Coastal Marsh." Ecological

Applications 13.5 (2003): 1400-16.

Bertness, Mark D., and Brain R. Silliman. "Shoreline Development Drives Invasion of P.

australis and the Loss of Plant Diversity on New England Salt Marshes."

Conservation Biology 18.5 (2004)

Chambers, Randolph M., Laura A. Meyerson, and Kristin Saltonstall. "Expansion of

P. australis australis into tidal wetlands of North America." Aquatic Botany.

Vol. 64. 1999. 261-73. Print.

Lachet, Standard Operating Procedure for Nitrate, Nitrite. 1995. Chicago, IL, Grace

Analytical Lab, 2nd Ed.

P. australis: Questions and Answers. U.S. Fish and Wildlife Service, Nov. 2007.

Web. 28 Oct. 2014. <http://www.fws.gov/gomcp/pdfs/

P. australisQA_factsheet.pdf>.

Samuel E. Hellings and John L. Gallagher. “The Effects of Salinity and Flooding on P.

australis.” Journal of Applied Ecology. Vol. 29, No. 1 (1992), pp. 41-49.

Strickland, J.D.H. and T.R. Parsons A practical handbook of Seawater Analysis 1972

Ottawa, Fisheries Research Board of Canada 2nd

. Ed.

Wilcove, David S., et al. "Leading Threats to Biodiversity: What's Imperiling

U.S. Species." Precious Heritage. Ed. Bruce A. Stein, Lynn S. Kutner, and

Jonathan S. Adams. New York: Oxford University Press, 2000. 239-55. Print.

Windham, Lisa Marie, and Laura A. Meyerson. "Effects of Common Reed

(Phragmites australis) Expansions on Nitrogen Dynamics of Tidal Marshes of the

Northeastern US." Estuaries 26.2B (2003): 452-64.

Figure 1. Map of all six sites.

Figure 2. Percent of shoreline covered in all sites by P. australis and non-P. australis

segments in low (<18 ppt) and high (>18 ppt) salinity zones.

Table 1. The shoreline, percent cover, average salinity, and average nitrogen for each

estuary.

Figure 3. Percent shoreline cover of P. Australis over average estuary salinity.

Figure 4. Percent of shoreline covered in all sites by P. australis and non-P. australis

segments in low (<14 μM) and high (>14 μM) nitrogen zones.

Figure 5. Percent of shoreline covered by P. australis and non-P. australis segments in

low and high salinity and nitrogen levels.

Figure 6. Percent shoreline coverage of P. australis in an estuary over average estuary

nitrogen concentration.

Figure 7. Percent shoreline coverage of P. australis in an estuary in low salinity zones

over average estuary nitrogen concentration.

Figure 8. Carbon to nitrogen ratios of all observed plant species with standard error.

Figure 9. Carbon to nitrogen ratio of P. australis and all spartina species in high and low

nitrogen zones.

Figure 10. Carbon to nitrogen ratio of P. australis plants and non-P. australis plants over

nitrogen concentration.

Figure 11. Percent cover of P. australis over salinity.

Figure 12. Percent cover of S. alterniflora over salinity.

Figure 13. Percent cover of S. patens over salinity.

Figure 14. Percent cover of T. latifolia over salinity.

Figure 15. Percent cover of all spartina species over salinity.

Figure 16. Percent cover of P. australis over nitrogen concentration.

Figure 17. Percent cover of S. alterniflora over nitrogen concentration

Figure 18. Percent cover of S. patens over nitrogen concentration.

Figure 19. Average percent cover of salt marsh species in P. australis segments and non-

P. australis segments.

Figure 20. Species richness in different plant segments.