Nutrient and salinity controls on Phragmites australis invasion in. Cape Cod salt marsh ecosystems

Transcription

1 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

2 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

3 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 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

4 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

5 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 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.

6 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 μm. At Little Sippewissett marsh it was 52.5 μm. At West Falmouth Harbor it was μm. At Oyster Pond it was 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

7 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

8 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

9 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

10 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.

11 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): 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): 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 Print. Lachet, Standard Operating Procedure for Nitrate, Nitrite Chicago, IL, Grace Analytical Lab, 2nd Ed. P. australis: Questions and Answers. U.S. Fish and Wildlife Service, Nov Web. 28 Oct < 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 Strickland, J.D.H. and T.R. Parsons A practical handbook of Seawater Analysis 1972 Ottawa, Fisheries Research Board of Canada 2 nd. Ed. Wilcove, David S., et al. "Leading Threats to Biodiversity: What's Imperiling

12 U.S. Species." Precious Heritage. Ed. Bruce A. Stein, Lynn S. Kutner, and Jonathan S. Adams. New York: Oxford University Press, 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): 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.

13 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.

14 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.

15 Figure 5. Percent of shoreline covered by P. australis and non-p. australis segments in low and high salinity and nitrogen levels.

16 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.

17 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.

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

19 Figure 11. Percent cover of P. australis over salinity. Figure 12. Percent cover of S. alterniflora over salinity.

20 Figure 13. Percent cover of S. patens over salinity. Figure 14. Percent cover of T. latifolia over salinity.

21 Figure 15. Percent cover of all spartina species over salinity. Figure 16. Percent cover of P. australis over nitrogen concentration.

22 Figure 17. Percent cover of S. alterniflora over nitrogen concentration Figure 18. Percent cover of S. patens over nitrogen concentration.

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

24 Figure 20. Species richness in different plant segments.