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 managements issues have arisen 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.
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 that Spartina spp. (salt glands) and is therefore impacted by
the saltwater flushing that occurs in the Neponset marsh (reference).
The effects of changes in the geochemistry of the Neponset River and 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). 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). 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 (reference).
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 available pool of nutrients in pore water, the pH
and salinity of the soil, and further decreasing the systems exposure to saltwater flushing
(Ehrenfeld, 2003; newenglandwetlands.com). The density 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).
Changes in nutrient cycling can also result from changes in the biodiversity in the
marsh. A decrease in the effectiveness of nutrient cycling can result from a decrease in
biodiversity (Weathers and Ewing, 2013). The changes in nutrient cycling resulting from
changes in dominant vegetative species in the marsh can contribute to variations in the
microbial community (Ehrenfeld, 2003). Changes in the microbial community attributed
to changes in nutrient inputs further alter the geochemistry of the marsh.
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 soil 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 natural to
Spartina spp. include changes in ground shading. The taller, thicker biomass of
Phragmites australis reduces evaporation because of the increase in ground cover
(Ehrenfeld, 2003). The main physiological difference between the two species is the
presence of salt excretion glands in Spartina spp.; Phragmites australis isn’t able
translocate and excrete salt through its leaves.
***Most elements follow the same path as nutrient cycling and marshes have been
proposed as potential sites for phytoremediation (Weathers and Ewing, 2013; Weis and
Weis, 2004). Phrag sequesters more metals than Spartina because (Weis and Weis, 2004)
Removal of Phragmites australis may reduce the rate of primary production in the marsh
(Ehrenfeld), but it will definitely decrease phytoremediation of metals and other
potentially toxic elements (Weis).
***While few previous projects have been done to manage the growth of Phragmites
australis, the work done hasn’t had follow-up testing or observations taken to check on
the success of the project.
***Very little research has been done on geochemistry in the Neponset River salt marsh
and what has been done doesn’t include soil chemistry. Soil chemistry in the marsh may
be an extremely important factor contributing to the success of any restoration attempts
that will be made.
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
Robinson, M. (2002). Common Reed: An Invasive Wetland Plant Phragmites australis.
http://www.mass.gov/eea/docs/dcr/watersupply/lakepond/factsheet/phragmites.pd
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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. (2003). Metal uptake, transport and release by wetland plants:
Implications for phytoremediation and restoration. Environment International, 30,
685-700.
http://newenglandwetlands.com/projects/