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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.