Early Spring Potentially Harmful Phytoplankton in Bahía Concepción, Gulf of California

Potentially harmful dinoflagellates like Dinophysis caudata, and Alexandrium catenella were identified in Bahía Concepción (Gulf of California) during March 1993 and April 1994. After this, high paralytic shellfish poisoning (PSP) levels, up to 2400 MU 100 g^–1 in whole shellfish, mainly Argopecten circularis, were measured. In the winter, before the presence of potentially harmful dinoflagellates, the hydrobiological structure was characterized by a diatom-dominated community, a well-mixed water column, and the seasonal decrease in water temperature. The disappearance of diatoms, an increase in water temperature, and the beginning of thermal stratification, were the main features associated with dinoflagellate dominance in early spring. A narrow temperature range of 20 to 23°C, lasting only a short time, rather than thermal vertical stability of the water mass, was found to be the main factor related to dominance of dinoflagellates. According to published literature, Alexandrium catenella can be responsible for PSP activity however, the presence of D. caudata would also suggest diarrhetic shellfish poisoning (DSP), recently confirmed by HPLC toxin analyses. Baja California Sur experiences around 300,000 diarrhea cases annually and it is difficult to assess how many of these cases could be related to phycotoxic activity.