ABSTRACT: Reduced grazing on harmful algal bloom species has been attributed to both the feeding deterrence and toxicity of the algae. Both toxic and deterrent effects of dinoflagellates of the genus Alexandrium, which contain toxins that causeparalytic shellfish poisoning, have been reported on different copepod species. We examined how toxin-containing Alexandrium fundyense affected ingestion rates of 2 geographically distinct Acartia hudsonica (Copepoda: Calanoida) populationsover short timescales. The copepod population from Great Bay, New Jersey, has never been exposed to blooms of toxic A. fundyense, whereas the population from Casco Bay, Maine, has experienced regular blooms of the highly toxic dinoflagellate fordecades. Our goals were to examine the mechanisms by which toxin-containing Alexandrium reduced the ingestion rates of copepods and determine whether the mechanisms were related to the exposure history of copepod populations. Copepods were fed, for48 h durations, sole diets of toxin-containing A. fundyense, non-toxin containing Alexandrium tamarense and the green flagellate Tetraselmis sp. (not known to have toxic effects) and different mixtures of each (70%Alexandrium/30% Tetraselmis sp., 40/60, 20/80). Changes in ingestion rates over time were determined by measuring ingestion at different time intervals (3, 6, 12, 24, 48 h) over the 48 h period using 3 h incubations. The naïve copepods fromNew Jersey initially ingested toxin-containing A. fundyense, in both sole and mixed diets, at high rates followed by decreases in ingestion over time. By 24 h, their total ingestion rates were near zero. The decreases in ingestion rates, which werenot due to prey selection, were also accompanied by reduced respiration rates. In contrast, none of these effects were observed when the New Jersey copepods fed on non-toxic A. tamarense. Additionally, feeding rates of historically exposedcopepods from Maine were not affected by the toxin-containing A. fundyense diets. These observations are consistent with the hypothesis that toxic A. fundyense physiologically incapacitated the copepods from New Jersey but not those fromMaine. Such toxic effects, as opposed to deterrence effects, can have profound implications on the grazers¹ ability to control harmful algal blooms.