Effects of nitrate and phosphate on grazer‐induced toxin production in Alexandrium minutum

Abstract

Two strains of Alexandrium minutum were exposed to waterborne cues from copepod grazers in factorial combinations of nitrate and phosphate concentrations to evaluate the importance of grazer‐induced paralytic shellfish toxin (PST) production under different nutrient regimes. In nitrate‐rich treatments, the presence of waterborne cues from grazers resulted in significantly increased cell‐specific PST content. In low nitrate treatments, however, waterborne cues from grazers did not result in any detectable increase in cell‐specific PST content. The grazer‐induced increase in cell‐specific PST content in nitrate‐rich treatments was comparable in size to the effects of nitrate in the absence of grazers. Effects of phosphate limitation were less consistent, resulting in increased PST content in nitrate‐rich treatments in one of the A. minutum strains, but had no significant effect in the other. The ability of dinoflagellates to sense and respond to the presence of grazers by increased PST production may be one of the most important factors affecting cell‐specific PST content under nitrogen replete conditions.