Environmental Factors and Seasonal Occurrence of the DinoflagellateAlexandrium minutum, a PSP Potential Producer, in a Mediterranean Lagoon

Seasonal fluctuations of hydrobiological parameters were studied, throughout 1992, in a shallow Mediterranean area (Ganzirri lagoon), affected by the dinoflagellateAlexandrium minutum, a potential producer of saxitoxins. Concentrations of dissolved inorganic nitrogen (DIN) and N:P ratios were high in autumn and winter, when the waters were well mixed and free fromA. minutumcells. The spring appearance of this species coincided with enhanced rainfall and freshwater runoff and with stabilization of the water column, whereas cell dispersion was observed in summer. AlthoughA. minutumnever dominated the algal community, cell densities were maximal in May, when values of salinity did not exceed 30; during this month, there were reduced amounts of DIN, N:P atomic ratios close to 16:1 and a minor competitive pressure by other phytoplankters. Subsequent increases in values of the above parameters—in particular, of the ammonium nitrogen—were associated with a decline in cell numbers, and final disappearance ofA. minutumcells.