Peggy Rimmelin

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

Abstract A 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.

A seasonal study of diatom dynamics in the North Atlantic during the POMME experiment (2001): Evidence for Si limitation of the spring bloom

(1) The Si cycle in the northeast Atlantic (40� -43.5� N, 15� -21� W) was investigated in the winter, spring, and late summer during the Programme Ocean Multidisciplinaire Meso Echelle (POMME) (2001). The aim of this study was to determine the principal causes of the onset and subsequent decline of the diatom bloom, with an emphasis on nutritional limitation processes. The siliceous compartment dynamics was characterized through silicic acid distribution, size-fractionated biogenic silica and Si uptake rates, Si uptake kinetics (KS and Vmax), and export rates. The results of the POMME cruises indicated an important seasonal variability of the siliceous compartment in the North Atlantic together with a spatial variability following a south-north gradient in parallel to the increasing stock of nutrients. Here we present the first kinetic evidences for Si limitation of the diatom spring bloom in the northeast Atlantic, supporting previous reports of potentially limiting silicic acid concentrations in this region. Integrated Si uptake rates were very low throughout the survey, except at the northern anticyclonic eddy site, where a Pseudo-nitzschia bloom was observed during spring and ranged between 0.04 and 11.2 mmol m � 2 d � 1 , which is comparable to values obtained in oligotrophic regions. The overall annual Si production budget for the entire POMME area (375,000 km 2 ) was extrapolated to 9.8 � 10 9 mol Si yr � 1 , while Si export fluxes at 400 m only represented 3% of the surface production.