Abstract
Environmental conditions ultimately leading to blooms of the toxic dinoflagellate Alexandrium catenella/tamarense were investigated at two Mediterranean sites (Annaba Bay, Algeria and Thau lagoon, France). Three years were examined in details: 1992 (a pre-Alexandrium period), 2002 (a year with the first bloom in Annaba) and 2010 (a year with a major bloom in Annaba). Most conditions were similar, but ammonium concentrations were much higher in Annaba (up to 100 μM) than in Thau (up to 10 μM). First records of A. catenella/tamarense were in 1995 for Thau and 2002 for Annaba, and coincided with soluble reactive phosphorus (SRP) decreasing below a concentration of about 1 μM. No other environmental variable could be related to those blooms. Thus, it is likely that the large reductions in SRP at both sites led to phosphorus limitation of a certain number of phytoplankton species and favored the development of A. catenella/tamarense.