Salt marshes are enriched when phosphorus and/or nitrogen flow into waters that ultimately flood the salt marsh. Agricultural fertilizers applied to fields throughout coastal watersheds move downslope into waters that flow toward salt marshes. Because many salt marshes are nitrogen-limited, the effect is to increase the productivity of both algae and vascular plants. Increased nitrogen loading stimulates algal growth, especially of green macroalgae, which form large mats that can smother vascular plants and benthic invertebrates. Indirect degradation occurs when microbial decomposition increases oxygen demand, causing soil hypoxia or anoxia and sulfide toxicity.
I. Valiela's long-term eutrophication experiment in a New England salt marsh indicates that nitrogen addition shifts S. alterniflora to S. patens and increases competition for light. Such altered competitive relationships are likely widespread, especially where considerable nitrogen is deposited from the air (e.g., from dairy operations in the Netherlands).
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