ENVIRONMENTAL-FACTORS AND SEASONAL OCCURRENCE OF THE DINOFLAGELLATE ALEXANDRIUM-MINUTUM, A PSP POTENTIAL PRODUCER, IN A MEDITERRANEAN LAGOON

Seasonal fluctuations of hydrobiological parameters were studied, thro ughout 1992, in a shallow Mediterranean area (Ganzirri lagoon), affect ed by the dinoflagellate Alexandrium 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 mi xed and free from A. minutum cells. The spring appearance of this spec ies coincided with enhanced rainfall and freshwater runoff and with st abilization of the water column, whereas cell dispersion was observed in summer. Although A. minutum never dominated the algal community, ce ll densities were maximal in May, when values of salinity did not exce ed 30; during this month, there were reduced amounts of DIN, N:P atomi c ratios close to 16:1 and a minor competitive pressure by other phyto plankters. Subsequent increases in values of the above parameters-in p articular, of the ammonium nitrogen-were associated with a decline in cell numbers, and final disappearance of A. minutum cells. (C) 1996 Ac ademic Press Limited