Nitrogen enrichment and sea-level rise are major and intensifying agents of ecological change in coastal ecosystems, including salt marshes. We propose to define long-term effects and interactions of chronic eutrophication and sea-level rise within experimental plots in the Great Sippewissett marsh in Eastern Massachusetts. Since 1970, we have been mapping vegetation and elevation within 10 m radius plots experimentally fertilized with 4 dosages of nitrogen. Preliminary data show continuing long-term shifts in form, cover, biomass and spatial distribution due to both eutrophication and sea-level rise. In un-enriched plots low marsh vegetation expanded at the expense of high marsh vegetation in response to the 12-14 cm sea-level rise during the 33 years of our studies. In fertilized plots, in contrast, increased nitrogen supply stimulated growth of high marsh plants whose thick thatch increased organic matter accumulation and favored capture of sediment particles from flooding water, thus countering submergence. We hypothesize that increased nitrogen supply and rising sea-level alter direction of competitive success among different species of marsh plants, and force long-term interactions that thoroughly reshuffle spatial disposition and composition of swards of salt marsh plants, and dependent animal communities. Continuing study of these plots offers an unmatched opportunity to understand how the interaction of chronic nitrogen enrichment and increased sea-level rise that is occurring in estuaries world-wide will restructure salt marsh ecosystems. We will also upload and analyze the extensive past data record for these plots for publication of long-term impacts, and sharing with the other researchers.
