One-dimensional model of short-term dynamics of the pelagic ecosystem in the NW Mediterranean Sea: effects of wind events

Abstract

A one-dimensional coupled physical–biological model of the mixed layer was applied to the pluridisciplinary data set acquired at short-time scales at a fixed station during the DYNAPROC cruise (May 1995) in the open NW Mediterranean. This cruise provided the opportunity to observe upper layer mixing, pycnocline deepening and vertical advection resulting from several wind events. The physical model is an eddy–kinetic-energy model; parameterisation of Ekman pumping and relaxation to the temperature data were introduced to reproduce the vertical advection observed during the major wind event. Eight variables were included in the ecosystem model: three classes of phytoplankton (diatoms, nanoflagellates and picoplankton), two nutrients (nitrate and ammonium), one herbivorous zooplankton and two types of detritus. Simulation over a 1-month period, performed with real meteorological data recorded during the cruise, reasonably reproduced the effects of the wind events, i.e. mixing and Ekman pumping which induced a 10-m vertical advection of the pycnocline. The observed nitrate and ammonium concentrations and the phytoplankton decrease were reproduced by the model. The transition from a mesotrophic ecosystem, characteristic of the spring bloom period, to an oligotrophic system was successfully simulated. For example, the nanoflagellate and diatom contributions decreased during the month while that of the picoplankton doubled; the decline in herbivorous zooplankton was also reproduced. The simulated results showed that the decrease in diatoms and nanoflagellates resulted from both grazing pressure and significant cloud cover at the end of the month. Vertical advection also plays a nonnegligible role on the simulated temporal changes of the three phytoplankton classes. In fact, it displaces the subsurface peak nearer the surface, and thus allows the phytoplankton to slightly develop a few days after the major wind event, even if diatoms and nanoflagellates decrease overall.