Print PageWe investigate global change at the land-sea margin, specifically salt marshes’ function as a “coastal filter” in central California, intercepting watershed-derived nitrogen (N) pollution. The uptake of N is understood to buffer the major coastal problem of eutrophication, and at the same time, sea-level rise may change this important marsh function. Nitrogen pollution and sea-level rise both impact coastal ecosystems, yet their interacting effects are poorly understood. In an experiment crossing simulated sea-level change and N addition, N addition had a significant, positive effect on marsh-plant growth, tissue quality, and total N sequestered, suggesting that coastal salt marsh plants serve as a robust N-trap and coastal filter; this function is not saturated despite extremely high background annual N inputs from agriculture. Our next research question was “Do findings – of an unsaturated capacity of salt marsh plants to take up N pollution – extend to the landscape scale around the estuary of Elkhorn Slough?” Expanding the study to the scale of the whole estuary, we quantified plant growth, tissue quality, and total N sequestered in 9 marshes in response to two factors: a gradient of total-N concentration in the estuary main channel (higher N/lower N) and the elevational gradient of the intertidal zone. Instead of manipulating plot heights to simulate sea-level rise, we used the intertidal elevational gradient for a range of inundation times; instead of adding inorganic N, we used the higher- and lower- N exposure during plants’ tidal inundation.
Results/Conclusions
We found contrasting results in the landscape-scale study relative to a field experiment: in this study, halophytes were saturated in their capacity to take up additional N in the estuary, both a) at the end of the rainy season (Spring), which is the peak delivery of nutrient pollution; and b) at the end of the summer (Fall), which is the peak growing season for marsh plants in Elkhorn Slough. Saturation was indicated as plants neither grew more nor sequestered more N in response to higher N inputs. Additionally, in the Spring harvest, plant root growth decreased significantly with higher N exposure; root biomass is a major component of maintaining the elevation of the marsh platform relative to mean tide. Also in contrast to the field experiment, plants at lower-N sites had significantly higher concentrations of N in tissues. We explore questions of scale and applications to conservation management.
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