Eutrophication and sea-level rise will affect salt marsh extent and functioning, living marine resources, and delivery of ecosystem services in estuaries. I investigate the role of threatened salt marsh habitats as a “coastal filter” that improves water quality of runoff into the ocean through microbial denitrification. The estuarine ecosystem service of nitrogen pollution reduction is important; eutrophication (excessive nutrients leading to algal blooms, subsequent die-offs, and hypoxic conditions) is the leading environmental problem in US coastal waters. Sea level rise is predicted to diminish the distribution of salt marsh relative to bare mudflats, through marsh drowning. I compare denitrification rates in vegetated sediments and mudflats and record plant nitrogen uptake in order to map the ability of a central California estuary to buffer rising nitrogen loading over the next century. I achieve this via a) literature review of denitrification rates in marsh and mudflat in temperate regions; b) a manipulative field experiment in Elkhorn Slough National Estuarine Research Reserve (ESNERR), California, in which I followed plant species diversity, aboveground net primary production, and tissue nitrogen in response to manipulated tidal height and nitrogen fertilization. The field experiment allows me to document nitrogen uptake by salt marsh vegetation as it is adversely impacted by a range of simulated sea-level rise.
Results/Conclusions
Published studies comparing denitrification in temperate vegetated sediments (including salt marsh) and mudflats (n=6, lab and field studies) show higher rates in vegetated sediments. Vegetated sediments – a mix of submerged and emergent vegetation – removed 10-100% of nitrate, while mudflats removed 0-40% of nitrate. No published studies address this comparison on the US Pacific Coast. In ESNERR after one year, summer (peak) halophyte biomass responded significantly to both manipulated elevation and nitrogen loading. Summer biomass was greatest in plots with no elevation change and with inorganic N addition of 390 g N/m2-yr. Added nitrogen, elevation change, and their interaction all significantly affected halophyte tissue percent nitrogen. Winter biomass was greatest in plots raised 10 cm with inorganic N addition: but all winter biomasses were at least threefold smaller than summer. These results a) suggest that marsh plants can continue to take up excess nitrogen, in summer and in winter, but that seasonality matters; and b) underscore the importance of a potential threshold at which nitrogen is no longer a limiting nutrient and plant uptake is diminished.
