Felicia Woods
EEOS 660
November 18, 2014
Importance of Geochemistry in the Restoration
of the Neponset River Salt Marsh
Geochemical processes link abiotic and biotic components of the ecosystem, and
are important in the restoration of the Neponset River salt marsh since virtually every
process in an ecosystem is linked to elemental cycling (Weathers and Ewing, 2013).
Environmental management issues stem from either limited availability of nutrients or
too many nutrients, causing limits in primary production or eutrophication, respectively
(Weathers and Ewing, 2013; Turner et al, 2009). Phragmites australis is the dominant
plant in the area being considered for restoration, as well as a problem in most of the
wetlands along the Neponset River watershed.
The common reed, Phragmites australis, is extremely successful at overrunning
areas that have been disturbed by either natural or anthropogenic sources (Robinson,
2002). Because of the relative size of the plant, Phragmites australis has a high nitrogen
demand that supports higher rates of primary production than native Spartina spp.
(Ehrenfeld, 2003). Although it can tolerate brackish water to some extent, it does not
have the adaptive mechanism of Spartina spp. (salt glands) and is therefore impacted by
the saltwater flushing that occurs in the Neponset River marsh (Weis and Weis, 2004;
newenglandwetlands.com/projects).
The effects of changes in the geochemistry of the Neponset River marsh have
benefited the growth of Phragmites australis. Sources of excess nutrients and sediments
that have contributed to the success of Phragmites australis include nutrient pollution in
the Neponset River watershed from runoff, and displacement of sediment from ditching
and dredging (newenglandwetlands.com/projects). The sediment that was removed
during ditching, to allow for better drainage, was piled along the banks of the ditches and
elsewhere
in
the
marsh,
increasing
the
elevation
of
the
marsh
platform
(newenglandwetlands.com/projects). The increase in elevation decreased the effect of the
natural cycle of saltwater flushing that takes place within the marsh and therefore began
to change the geochemistry of the Neponset salt marsh (newenglandwetlands.com
/projects).
Ditching the marsh has not only increased the elevation of the marsh and
decreased the impact of saltwater flushing, it also increased the area exposed to nutrient
pollution in the water. Nutrient loading from runoff and sewage leaks have dramatically
increased the availability of nitrogen and helps meet the high nitrogen demand of
Phragmites australis while contributing to an increase in aboveground biomass (Deegan
et al, 2012). Turner et al (2009) explained the effect that eutrophication has on the
stability of vegetation within the marsh because it compromises the integrity of the root
matrix. Increasing the nutrients available to the plants alters their stability by increasing
the shoot-to-root ratio (Deegan et al, 2012). As the aboveground biomass increases, and
plants become top-heavy, vegetation in the marsh is prone to uprooting and erosion
(Turner et al, 2009).
The effects that Phragmites australis has potentially had on the geochemistry in
the Neponset marsh include changing the pool of nutrients in pore water, the pH and
salinity of the soil, and further decreasing the systems exposure to saltwater flushing
Woods 2
(Ehrenfeld, 2003; newenglandwetlands.com/projects). The density of the Phragmites
stems traps a considerable amount of sediment, further increasing the elevation and
decreasing saltwater flushing. The reductions in water flow and flood retention caused by
the increased sediment retention further impact nutrient cycling (Ehrenfeld, 2003).
The area in consideration has become a monoculture of Phragmites australis and
changes in nutrient cycling can also result from changes in the biodiversity in the marsh
(Ehrenfeld, 2003; Neponset River Watershed Wetlands Restoration Plan, 2000). A
decrease in the effectiveness of nutrient cycling can result from a decrease in biodiversity
(Weathers and Ewing, 2013). For example, carbon fluxes associated with the deposition
of litter typically vary with different species of plant. While the stems of Phragmites
australis decompose slower than those of Spartina spp., the leaves decompose faster
(Ehrenfeld, 2003). The changes in nutrient cycling resulting from differences in the
dominant vegetative species in the marsh can contribute to variations in the microbial
community (Ehrenfeld, 2003). Changes in the microbial community attributed to
fluctuations in nutrient quality and quantity further alter the geochemistry of the marsh
(Ehrenfeld, 2003).
Both physical and physiological differences in Phragmites australis and Spartina
spp. contribute to changes in soil salinity (Ehrenfeld, 2003). Physical differences
contributing to changes in soil salinity when Phragmites australis invades areas native to
Spartina spp. are due to changes in ground shading. The taller, thicker biomass of
Phragmites australis reduces evaporation in the soil because of the increase in ground
cover and therefore decreases salinity (Ehrenfeld, 2003). The main physiological
difference between the two species is the presence of salt excretion glands in Spartina
Woods 3
spp.; Phragmites australis isn’t able translocate and excrete salt through its leaves
(Ehrenfeld, 2003).
The high nitrogen uptake of Phragmites australis increases denitrification rates in
brackish marshes, such as the Neponset salt marsh, and affects the nitrogen pools in pore
water (Ehrenfeld, 2003). By decreasing the pool of ammonium (NH4+) in pore water, it
also contributes to changes in the pH of the surface soil of the marsh (Ehrenfeld, 2003).
However, Phragmites is used in some wetlands to treat wastewater contaminated with
metals because of it ability to take up metals from soil (Weis and Weis, 2004). Although
this may not be the case in the Neponset River marsh (no data was found on it), it may
have an impact on the ecosystem if Phragmites is completely removed.
While the presence of Phragmites australis may cause changes in the
geochemistry of the Neponset River marsh, the removal of it could also dramatically
impact the ecosystem. Since most elements follow the same path as nutrient cycling,
marshes have been proposed as potential sites for phytoremediation (Weathers and
Ewing, 2013; Weis and Weis, 2004). Phragmites australis sequesters more metals than
Spartina spp. because it doesn’t release the toxic substances back into the marsh
environment through leaf excretion (Weis and Weis, 2004). Although replacing
Phragmites with Spartina would remove the metal contaminants from the soil, it isn’t
able to contain the contaminants like Phragmites (Weis and Weis, 2004). However, more
research needs to be done to explore the effect of the translocation of the pollutants, and
what complications could arise from the decomposition of the below ground biomass of
Phragmites (Weis and Weis, 2004).
Woods 4
Removal of Phragmites australis may reduce the rate of primary production in
the marsh (Ehrenfeld, 2003), but it will definitely decrease phytoremediation of metals
and other potentially toxic elements (Weis and Weis, 2004). Another impact that the
removal of Phragmites australis would have on the geochemistry of the Neponset River
marsh would be the increase of biodiversity because of the decreased competition for
space. However, little has been done to manage the regrowth of Phragmites australis in
areas that it has been removed in restoration attempts and, if left unchecked, it can
quickly take over again.
Some restoration work was done on a section of the marsh in 2005 to manage the
growth of Phragmites australis along the Neponset River (newenglandwetlands.com
/projects). The work done by Great Meadows, LLC under the guidance of the
Massachusetts Department of Conservation and Recreation involved removing dredge
spoil to decrease the elevation of the marsh in the area (newenglandwetlands.com
/projects). The goal of the project was to once again expose the area to salt water flushing
in an attempt to return the area to a healthy marsh ecosystem. However, the work done on
the marsh has not had follow-up testing or observations taken to check on the progress of
the project.
Very little research has been done on geochemistry in the Neponset River marsh,
and what research has been done doesn’t include soil chemistry. Even though water
chemistry is important to ecosystem functions, the soil chemistry in the Neponset marsh
can be an extremely important factor contributing to the success of any restoration
attempts that will be made. Understanding how, or if, soil composition differs in areas
dominated by Phragmites australis, compared to that of the soil in the more natural
Woods 5
Spartina spp. dominated areas, could be the basis of management via geochemistry.
However, even though the removal of the Phragmites will reduce the accumulation of
sediment and potentially promote Spartina regrowth, the removal would also decrease the
rate of primary production in the marsh, decreasing the rate of carbon sequestration.
Knowing how the geochemistry affects the growth of Phragmites australis and how it in
turn affects the geochemistry of the Neponset River marsh is the first step to planning a
successful restoration project.
Woods 6
References:
Deegan, L., Johnson, D., Warren, R., Peterson, B., Fleeger, J., Fagherazzi, S., &
Wollheim, W. (2012). Coastal eutrophication as a driver of salt marsh loss. Nature, 490,
388-392.
Ehrenfeld, J. (2003). Effects of Exotic Plant Invasion on Soil Nutrient Cycling
Processes. Ecosystems, (6), 503-523.
http://www.falw.vu.nl/en/images/ehrenfeld_tcm24-80025.pdf
Neponset River Watershed Wetlands Restoration Plan
http://www.mass.gov/envir/massbays/pdf/moris/neponset_wetlands_restoration_p
lan.pdf
Robinson, M. (2002). Common Reed: An Invasive Wetland Plant Phragmites australis.
http://www.mass.gov/eea/docs/dcr/watersupply/lakepond/factsheet/phragmites.pd
f
Turner, R., Howes, B., Teal, J., Milan, C., Swenson, E., & Goehringer-Toner, D. (2009).
Salt marshes and eutrophication: An unstable outcome. Limnology and
Oceanography, 54(5), 1634-1642.
Weathers, K., & Ewing, H. (2013). Element Cycling. In Fundamentals of Ecosystem
Science (pp. 97-108). Amsterdam: Academic Press/Elsevier
Weis, J., & Weis, P. (2004). Metal uptake, transport and release by wetland plants:
Implications for phytoremediation and restoration. Environment International, 30,
685-700.
http://newenglandwetlands.com/projects/
Woods 7