Ammonium and maintenance of bloom populations of Alexandrium fundyense in the Gulf of Maine and on Georges Bank: results of laboratory culture experiments

Blooms of the toxic dinoflagellate Alexandrium fundyense, responsible for paralytic shellfish poisoning, occur annually in the Gulf of Maine-Georges Bank region of the northwest Atlantic Ocean and often reach highest cell densities in surface waters depleted of nitrate, suggesting a dependence on recycled ammonium. We report here the results of batch culture experiments with A. fundyense designed to: (1) describe the kinetics of ammonium uptake and cell growth rates over a range of ammonium concentrations encompassing those observed in the Gulf of Maine Georges Bank region; (2) compare rates of nitrate and ammonium uptake in the presence of one another; and (3) determine whether growth rates of A. fundyense on ammonium at concentrations observed in the region are sufficient to maintain established bloom populations and the degree to which continued growth of the bloom populations may be sustained by ammonium. The resulting half-saturation constant for growth (K-s(g)) on ammonium was 0.93 mu M, and the half-saturation constant for ammonium uptake (K-s(N)) was 1.74 mu M. The maximum growth rate (mu(max)) was 0.51 d(-1), and the maximum cell-specific uptake rate (V-max) was 7.0 x 10(-5) mu mol NH4+-N cell(-1) d(-1). Based on these results we show that observed ammonium concentrations in the Gulf of Maine Georges Bank region during A. fundyense blooms are sufficient to sustain the blooms for extended periods (weeks) and will allow modest continued growth.