Selenium in plankton from the northwestern Mediterranean Sea

Selenium is a multiple oxidation state trace element whose redox chemistry and concentration in sea water can be modified by biological activity in the euphotic zone. A field study was undertaken to examine the effects of oceanographic and biological parameters on the selenium concentration of the coastal plankton. For this purpose, plankton was collected in the Ligurian Sea from October 1990 to December 1991. The samples analyzed were composed of mixed phytoplankton and microzooplankton ranging in size from 50 to 200 μm. The medium selenium concentration measured in microplankton was 1.57 μg Se•g−1 carbon or 0.62 μg Se•g−1 dry weight (n = 15). Seasonal fluctuations in selenium concentration of microplankton were influenced by hydrodynamic conditions. During strong vertical mixing of the water column, deeper water with high selenite and selenate concentrations may reach the surface and selenium concentration in plankton was found to increase by a factor of eight compared to the median annual value. Under usual hydrodynamic conditions, seasonal fluctuations of the microplankton selenium content were typically low. Such low variations appeared to be due to the specific composition of microplankton. In the case of a phytoplankton bloom, selenium concentration in microplankton increased in proportion to the diatom biomass, whereas in other cases a negative linear relationship was found between selenium content and zooplankton biomass. The biogeochemical cycling of selenium may be influenced by seasonal fluctuations of the plankton selenium content which is controlled by hydrodynamic processes and plankton composition.