SELENIUM IN PLANKTON FROM THE NORTHWESTERN MEDITERRANEAN-SEA

Selenium is a multiple oxidation state trace element whose redox chemi stry and concentration in sea water can be modified by biological acti vity in the euphotic zone. A field study was undertaken to examine the effects of oceanographic and biological parameters on the selenium co ncentration of the coastal plankton. For this purpose, plankton was co llected in the Ligurian Sea from October 1990 to December 1991. The sa mples analyzed were composed of mixed phytoplankton and microzooplankt on ranging in size from 50 to 200 mu m. The medium selenium concentrat ion measured in microplankton was 1.57 mu g Se . g(-1) carbon or 0.62 mu g Se g(-1) dry weight (n=15). Seasonal fluctuations in selenium con centration 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 s elenium concentration in plankton was found to increase by a factor of eight compared to the median annual value. Under usual hydrodynamic c onditions, seasonal fluctuations of the microplankton selenium content were typically low. Such low variations appeared to be due to the spe cific composition of microplankton. In the case of a phytoplankton blo om, selenium concentration in microplankton increased in proportion to the diatom biomass, whereas in other cases a negative linear relation ship was found between selenium content and zooplankton biomass. The b iogeochemical cycling of selenium may be influenced by seasonal fluctu ations of the plankton selenium content which is controlled by hydrody namic processes and plankton composition. Copyright (C) 1996 Elsevier Science Ltd