A NITROGEN-BASED MODEL OF PLANKTON DYNAMICS IN THE OCEANIC MIXED LAYER

In the initial development and exploration of the model's behaviour the authors have concentrated on modeling the annual cycle at Station "S' near Bermuda using 7 compartments (Phytoplankton, Zooplankton, Bacteria, Nitrate, Ammonium, Dissolved organic nitrogen and Detritus). Modelled cycles of zooplankton and bacterial stocks, and magnitudes of their annual production which cannot be validated due to sparse observations, are driven by the amplitude of the spring bloom and by changes in foodweb structure. Most model runs exhibit a spring bloom triggered by the winter depression of zooplankton stocks and the vernal increase in solar irradiance. The bloom is driven by nitrate entrained into the mixed layer during the wintertime deepening of the mixed layer. Following the shoaling of the pycnocline to c20 m, nitrate supply is limited to diffusional inputs, nitrate stocks are depleted, and regenerated production ewxceeds new production. The resulting cycles of new and regenerated production produce an annual cycle of the f-ratio with winter maxima approaching 0.8-0.9 and summer minima reaching c0.1-0.2, with annual values averaging 0.7. -from Authors