Abstract
Deep water convection (DC) in winter is one of the major processes driving open-ocean primary productivity in the Northwestern Mediterranean Sea. DC is highly variable in time, depending on the specific conditions (stratification, circulation and ocean-atmosphere interactions) of each specific winter. This variability also drives the interannual oscillations of open-ocean primary productivity in this important region for many commercially-important fish species. We use a coupled model system to 1) understand to what extent DC impacts phytoplankton seasonality in the present-day and 2) to explore potential changes in future scenarios (~2030). Our model represents quite accurately the present-day characteristics of DC and its importance for open-ocean phytoplankton blooms. However, for the future scenarios the importance of deep nutrients in fertilizing the euphotic layer of the NW Mediterranean decreases. The model simulates changes in surface density and on the levels of kinetic energy that make mesoscale activity associated with horizontal currents to become a more important fertilization mechanism, inducing subsequently phenological changes in seasonal plankton cycles. Because of our focus on the open-sea, an exact quantification of the impact of those changes on the overall biological production of the NW Mediterranean cannot be made at the moment.
Highlights
Among the general oligotrophy of the Mediterranean Sea[1,2] its north-western (NW) region presents mesotrophic conditions[3] linked with several sources of nutrients to the surface layer such as the Atlantic influx, riverine discharge, atmospheric deposition and deep ocean convection[4]
Deep convection (DC) events can be an important source of nutrients for the surface layer of the NW Mediterranean[15] and the nitrate and phosphate levels observed at surface during convective episodes can be very close to deep concentrations[16,17]
The temporal and spatial pattern of the mixed layer depth (MLD) in the NW Mediterranean shown in Fig. 2 indicates that the model is able to simulate the main characteristics of the DC in the region
Summary
Among the general oligotrophy of the Mediterranean Sea[1,2] its north-western (NW) region presents mesotrophic conditions[3] linked with several sources of nutrients to the surface layer such as the Atlantic influx, riverine discharge, atmospheric deposition and deep ocean convection[4]. DC events can be an important source of nutrients for the surface layer of the NW Mediterranean[15] and the nitrate and phosphate levels observed at surface during convective episodes can be very close to deep concentrations[16,17]. These nutrients inputs are larger than terrestrial[18] and atmospheric[19] inputs and could drive the strong seasonal cycle of phytoplankton observed in this region. The plankton community structure[29] and the phytoplankton surface abundance and horizontal distribution in spring[30] seem to be controlled by the duration, intensity and extension of the winter DC in the NW Mediterranean region[31]
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