Coastal ecosystems are experiencing changes in nutrient loading and species composition much higher than previously measured, undoubtedly affecting productivity and sustainability of coastal regions. Predicting the impacts of these changes is important in salt marshes because marshes are among the most biologically productive areas in the world and provide critical ecosystem services, such as nursery areas for fisheries, removal of nitrogen, and shoreline storm buffering. This project addresses a critical question: What are the effects of increased nutrients and species change on ecosystem services provided by salt marsh ecosystems? The project is the first to use whole-ecosystem experiments that alter nutrient loading and reduce fish populations to understand the effects on nutrient cycling, species diversity, food webs and the long-term sustainability of salt marshes.
The damage by Hurricane Katrina due to degradation of coastal wetlands is one example of the importance of salt marshes to society. A growing appreciation of this role for marshes has led to estuaries becoming the focus of large-scale restoration programs. This project will provide information essential for developing water quality standards and providing perspective for multi-million dollar coastal management decisions, such as construction of sewage treatment facilities, regulation of fishing, or rebuilding of lost marshes.
There is an extensive literature documenting multiple negative impacts of excess nutrients (N and P in various forms) on a range of estuarine ecosystem functions and services. Most such research has focused on water column and benthic communities. This project was designed to understand the impacts of chronic high nutrients on the other major component of estuarine ecosystems: tidal wetlands, specifically here, salt marshes. The prevailing view among marsh and estuarine ecologists is that salt marshes can absorb nutrients from flooding tidal waters without harm, thus marshes help buffer and protect the larger estuarine system against potential damages from nutrient pollution. Our research tested this prevailing hypothesis at an ecosystem level scale on the Plum Island Sound salt marshes in Ipswich and Rowley, MA. Whole creek sheds of ca. 6 ha were fertilized by adding concentrated nutrient solution to flooding tidal waters to a level (± 70µM) considered "moderately eutrophic". This approach mimicked the way marsh communities would naturally be exposed to excess nutrients. Most prior salt marsh fertilization experiments have used small plots (<1 to 10 m2) and dry fertilizer. Other results from this project do demonstrate that marshes can remove significant amounts of nitrogen from flooding tidal waters[1] [2]. Results from our work on the plant community itself, however, argue strongly that this is not without cost. The part of a salt marsh termed "low marsh" is marshland below ca. mean high water; flooded daily by the tides, and on the Atlantic coast of North America it is dominated by the grass Spartina alterniflora. Because of its twice daily tidal inundation, it is the area of salt marsh that interacts most with estuarine waters, and animals of the water column. It also has the greatest above ground primary production of the salt marsh plant communities: ca. 1300 – 1500 g m-2 in Plum Island Sound. This two – four m wide vegetation belt bordering creek banks began to collapse into creeks in large and small peat blocks. These would be flooded too much for the plants to survive, and eventually would erode away. After nine years it was clear that excessive nutrients in the fertilized creeks were driving the loss of creek bank low marsh, and the ecosystem functions and values this portion of the ecosystem provides. As expected, height and weight of individual S. alterniflora shoots increased somewhat in response to nutrient loading. Contrary to what might be predicted from the literature, however, lower stem densities resulted in no significant differences in above ground production between fertilized and reference low marsh. Further, lower below ground plant growth and greater microbial decomposition in fertilized creeks contributed to creek bank weakening and erosion. The literature also suggests that fertilization should drive replacement of high marsh (above mean high water) grasses by S. alterniflora. This was not observed in our study, even after nine years of fertilization. These and other results of the project have recently been published in the journal Nature[3] 1 Drake, D. C., L.A. Deegan, L.A. Harris, E.E. Miller, B.J. Peterson, R.S. Warren. 2008. Plant N dynamics in fertilized and natural New England salt marshes; a paired 15N tracer study. Marine Ecology Progress Series 354:35–46, 2 Drake, D. C., B.J. Peterson, K. A, Glavan, L.A. Deegan, C. Hopkinson, M.J. Johnson, K. Koop-Jacobsen, L.E. LeMay, and C.Picard. 2009. Salt marsh ecosystem biogeochemical responses to nutrient enrichment: a paired 15N tracer study. Ecology 90:2535-2546. 3 Deegan L.A., D.S. Johnson, R. S. Warren, B. J. Peterson, J. W. Fleeger, S. Fagherazzi, and W. M. Wolheim. 2012. Coastal Eutrophication as a Driver of Salt Marsh Loss. Nature 490:388 - 392.
