A Simple Heuristic Model of Nutrient Cycling in an Estuary
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Three decades of discussion and study have not resolved the apparent discrepancy between N-limitation of primary production and the ability of marine ecosystems to fix N. We use a simple model as a heuristic tool to examine controls on nutrient cycling in a shallow estuary, with Tomales Bay, California as the prototype. The modes is a steady-state, one-box model with inputs and losses of nutrients and organic matter, and terms representing N-fixation and dentrification. The physical description of the system is deliberately kept simple to permit a focus on the key biogeochemical reactions. Growth of autotrophs in the model can be limited either by dissolved inorganic nitrogen (DIN) or dissolved inorganic phosphorus (DIP). Nitrogen-fixation is controlled by the availability of DIP or limited by excess amounts of DIN. Model results demonstrate that, for a system with a long residence time, autotroph biomass and total organic matter are controlled primarily by the rate of delivery of P to the system, either as DIP or in organic matter.