Silicon cycle in the Northwestern Mediterranean Sea: seasonal study of a coastal oligotrophic site.

International audienceA study of the biogeochemical cycle of silicon has been conducted in the Gulf of Lion (NW Mediterranean) from September 1999 to September 2000. Most of the year the study site was under the influence of the NW Mediterranean Current, characterized by oligotrophic conditions. A seasonal pattern of silicon stocks was found, showing an inverse annual distribution of biogenic silica and lithogenic silica. Biogenic silica integrated stocks were higher during spring and summer (21.5 and 19.3 mmol m-2) due to siliceous phytoplankton Si uptake and build-up of biomass. By contrast lithogenic silica integrated stocks were highest during the fall and winter (61.8 and 45.0 mmol m-2), which may be explained by a higher degree of turbulence of the water column, inducing sediment resuspension. Phytoplankton counts showed that the relative contribution of diatoms to microphytoplankton at the chlorophyll a maximum averaged 51% during the study period. Si uptake rates, measured in situ from March to September 2000, were low (Σ ρSi = 0.14-1.4 mmol Si m-2 d-1) throughout the study period. Potential Si limitation of siliceous phytoplankton in the course of spring bloom development was evidenced both by Si enrichment kinetics, yielding relatively high KS values (3.46 and 4.97 μM), and by nutrient distributions, exhibiting Si exhaustion over the entire water column by mid-April. The annual integrated Si production rate amounted to 0.14 mol Si m-2 per year, one of the lowest rates reported to date, and the diatom contribution to annual carbon primary production was estimated to range between 24 and 36%. The silicon cycle at the NW Mediterranean site was similar to that observed in other oligotrophic open-ocean systems in terms of stocks, annual Si production rates, and the relative contribution of diatoms to phytoplanktonic primary production