The role of inorganic nutrients and dissolved organic phosphorus in the phytoplankton dynamics of a Mediterranean bay

The effect of Dissolved Organic Phosphorus (DOP) availability and nutrient limitation of phytoplankton growth in an estuarine bay (Alfacs Bay, NW Mediterranean) have been studied by means of a zero-dimensional ecological model including nitrogen, phosphorus (organic and inorganic), two groups of phytoplankton (diatoms and flagellates), one group of zooplankton, and detritus. Simulations with and without DOP as an extra source of phosphorus for phytoplankton growth suggest that DOP plays an important role in the dynamics of the Alfacs Bay ecosystem. DOP is indeed necessary to simulate the observed draw-down of nitrate and build up of phytoplankton biomass. Two non-exclusive mechanisms allowing DOP availability for phytoplankton are possible: direct uptake, or remineralization to Dissolved Inorganic Phosphorus. Including both gives a better agreement with the observations. Inclusion of DOP in the model leads to predominance of phosphorus limitation of phytoplankton growth in fall and winter, and of nitrogen limitation in late spring and summer. Simulations with and without sediment resuspension suggest that this process does not significantly affect the nutrient budget in the bay.