Experimental and regional studies of sources of nitrogen using models and stable isotopes in salt marshes

I examined the fate of nitrogen entering coastal environments, at three different spatial scales. Development and land use changes have increased the areas affected by eutrophication world wide. To monitor eutrophication and mitigate its effects, we need to assess changes in inputs over time and pinpoint the sources of N entering coastal ecosystems.;I first investigated the fate of externally-derived N in 314m 2 experimentally fertilized plots in Cape Cod's Great Sippewissett marsh. Vertical profiles of delta15N (a tracer of N sources and N transformations) revealed that delta15N increased as a result of N addition; these increases were smaller than the N added experimentally. The small degree of N burial related to N load, since 33 -- 75% of fertilizer N added was denitrified.;Second, I compared vertical profiles of delta15N in salt marsh sediments in a series of estuaries within Cape Cod's Waquoit Bay estuarine system. From 1940 -- 2005 these subestuaries received different land-derived N loads from their watersheds. Salt marsh sediment delta 15N increased significantly with increasing N load. The relationship of N supply and delta15N within marsh sediment at the ∼100m 2 scale and the subestuary-scale were reasonably similar. This suggests that it is feasible to scale-up results obtained in small-scale experiments to larger spatial units. Spartina alterniflora delta 15N was a more sensitive indicator of N loads than salt marsh sediments. delta 15N of S. alterniflora increased with estuarine N loads and increased more at lower N loads than at higher N loads, both in a space-for-time comparison as well as a real-time comparison.;Third, at even larger spatial scales (∼1000s km2), I defined linkages between N loads and N sources. Although within-watershed retention was high (77%), land-cover changes increasingly force enough through-put of land-derived N to enrich coastal waters. As land-cover further shifts away from natural vegetation, salt marshes will receive higher N loads. Some of this N will be buried or denitrified, but some will reach receiving waters, so that eutrophication of GSB will likely increase.