Responses of a Major Land Margin Ecosystem to Changes in Terrestrial Nutrient Inputs: Internal Nutrient Cycling, Production and Export

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The study of nutrient cycling processes and their relation to autotrophic production is of fundamental importance in the analysis of natural ecosystems. While it is axiomatic that aquatic ecosystems at the land-sea interface are among the most productive in the biosphere, surprisingly little is known about nutrient inputs from land, fate and cycling of these nutrients, and associated organic production. Recent analyses have suggested that estuaries may be generally heterotrophic (gross primary production is less than community respiration), implying that they are nutrient sources to the sea rather than sinks of terrestrial inputs. For the proposed research program, a cohesive interdisciplinary team of ecologists, geochemists, modelers and oceanographers has been assembled to address two hypotheses: 1) Chesapeake Bay, as an example of a coastal plain estuary, supports a N-limited autotrophic (GPP exceeds R) community which exports DON offshore as a consequence of internal cycling and transformation processes; and 2) spring inputs of N from land are coupled to the summer productivity maximum by phytoplankton assimilation and biomass deposition in spring with subsequent benthic regeneration and vertical transport during summer. These hypotheses will be tested in this 5-year project by making regular measurements of hydrographic variables and key carbon and nutrient processes at stations along the main salinity gradient and at times emphasizing spring and summer periods. Estuaries, in this cases especially the Chesapeake Bay, are important zones of ecological interaction mediating the exchange processes of continents and oceans. They provide a broad array of exploitable resources as fragile as the estuarine system itself. Research such as this will allow the more informed management of coastal ecosystems.