Toxic and noxious phytoplankton in Big Glory Bay,Stewart Island,New Zealand

Diurnal vertical profile sampling of the water column, during a fish killing bloom of the raphidophycean alga Heterosigma akashiwo, revealed a phytoplankton population otherwise composed almost entirely of a variety of dinoflagellates. Of these Glenodinium danicum, Dinophysis acuta, Polykrikos schwartzii, Ceratium furca and Gyrodinium spirale were predominant. The distribution of the major species within the phytoplankton were documented and evidence of synchronous vertical migration of H. akashiwo, G. danicum and P. schwartzii was observed. Extracts of shellfish obtained during the bloom and tested by mouse bioassay showed no PSP toxicity but a marginal degree of DSP toxicity. During a subsequent one year phytoplankton monitoring programme another potentially noxious species (Chaetoceros convolutus) appeared and the seasonal reoccurrence of species present during the bloom (e.g. H. akashiwo) was observed. Important year to year differences in the summer phytoplankton (diatom versus flagellate dominated populations) were apparent and analysis of climate data showed that these differences related to different weather conditions prevailing during the two summer periods sampled. The data suggest the fish killing bloom was giving a chance to develop by a prolonged period of warm, calm weather (during which several heavy rainfall events occurred) leading to stable hydrographic conditions (i.e. stratification) and an increase in the retention time of water within the bay.