Siliceous phytoplankton production and export related to trans‐frontal dynamics of the Almeria‐Oran frontal system (western Mediterranean Sea) during winter

A study of the biogeochemical properties of the Almeria‐Oran front was carried out in December 1997 to January 1998. A strong salinity gradient between Atlantic and Mediterranean waters in the Alboran Sea allowed the differentiation of several subsystems: the Mediterranean waters, the frontal zone, and the anticyclonic gyre. Si and C biomass and production were clearly enhanced by the frontal dynamics on the Atlantic side of the jet while Mediterranean waters, which encountered severe nutrient depletion in the mixed layer, exhibited a typical oligotrophic regime. The distribution of particulate matter was controlled by a cross‐frontal downwelling along the isopycnal slopes, that shoaled to the surface on the dense Mediterranean side and deepened toward the Atlantic side of the jet. A strong decoupling of production and biomass maximums occurred between the frontal limit, where particulate matter was produced, and the gyre, where it was accumulated. Export fluxes at 300 m were low at the frontal limit, representing 1–2% of surface Si and C production, and it is hypothesized that advective fluxes rather than grazing were the main factor limiting the accumulation of biomass. The adjacent systems, namely the associated anticyclonic gyre and the Mediterranean waters, were exporting Si to depth more efficiently than the frontal zone. The Si and C decoupling with depth appeared higher in the Almeria‐Oran frontal system than in other open‐ocean zones. The integrated Si production at the Almeria‐Oran Front was 0.83 mmol Si m −2 d −1 , which was closest to the production rates of mid‐ocean oligotrophic gyres than of other frontal systems, and may be explained by the sampling period, which occurred in the winter season.