A Simple Heuristic Model of Nutrient Cycling in an Estuary

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 model is a steady-state, one-box model with inputs and losses of nutrients and organic matter, and terms representing N-fixation and denitrification. 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. Increasing the delivery rate of DIN raises autotroph biomass slightly but has little effect on total organic matter. This is because the rates of input of P as DIP or organic matter control the N-fixation rate, and denitrification limits the build-up of DIN in the system. Thus, denitrification and N-fixation act as opposing negative feedbacks, insuring that the supply of N remains roughly commensurate with that of P. When exchange with the ocean is increased, reducing residence time, the relative importance of DIN input increases relative to that of DIP.