Growth responses of Alexandrium minutum (Dinophyceae) as a function of three different nitrogen sources and irradiance

The growth response of the PSP-producing dinoflagellate Alexandrium minutum, cultured from a shellfish poisoning outbreak in 1993 in the Bay of Plenty, was studied in semi-continuous cultures, using 54 combinations of irradiance and different nitrogen (N) sources (nitrate (NO3-), ammonium (NH4+), urea) and concentrations. Irradiance had more influence on growth of A. minutum cultures enriched with NO3- than on cultures enriched with either NH4+ or urea. At the high irradiance level (100 mu mol m(-2) s(-1)), the greatest growth (0.5 doubling d(-1)) was recorded in cultures enriched with NO3-, followed by cultures enriched with NH4+ and urea, respectively. For cultures enriched with either NH4+ or urea, nutrient concentrations contributed to greater variations in growth than irradiance. At high irradiance, growth of A. minutum peaked in cultures enriched with about 25 mu M of either NH4+-N or urea-N. Growth of A. minutum was substantially reduced when grown at 50 mu M of either form of ''recycled'' N, and growth ceased at concentrations greater than or equal to 100 and greater than or equal to 200 mu M of NH4+-N and urea-N, respectively. Growth of A. minutum appeared to saturate at relatively low irradiance (I-k < 40 mu mol m(-2) s(-1)). These results imply that A. minutum is able to substain reasonably good growth rates, even in poorly illuminated depths within the water column.