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

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Abstract
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 (NH+ 4), 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 μmol 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 μM of either NH4 +‐N or urea‐N. Growth of A. minutum was substantially reduced when grown at 50 μM of either form of “recycled” N, and growth ceased at concentrations ≥ 100 and ≥ 200 μM of NH4 +‐N and urea‐N, respectively. Growth of A. minutum appeared to saturate at relatively low irradiance (I k < 40 μmol 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.