Mechanisms, ecology, and toxicology of zooplankton feeding processes in the presence of harmful algae (Alexandrium spp.)

The role of grazers in the population dynamics of toxic Alexandrium spp., and specific mechanisms of feeding responses of zooplankton to toxic prey, have not been well studied. I had three main objectives in this study: to determine whether copepods could recognize and selectively avoid toxic Alexandrium spp. prey; to assess whether selective grazing in natural field bloom conditions could affect Alexandrium spp. bloom dynamics; to quantify toxin uptake, retention, and detoxification in copepods that fed upon Alexandrium spp. dinoflagellates. Experiments with fluorescently labeled toxic and non-toxic strains of Alexandrium spp. demonstrated that copepods could distinguish cell types on the basis of toxin content. Feeding trials with mixtures of Alexandrium spp. and non-toxic alternate dinoflagellate species demonstrated that non-toxic Alexandrium spp. were selectively ingested, and toxic Alexandrium spp. were selectively avoided. Experiments were conducted with field collected copepods and water samples containing natural concentrations of toxic Alexandrium fundyense. Results demonstrated that at the low levels of Alexandrium spp. abundance typical of the southwestern Gulf of Maine (∼1000 cells liter−1), toxic cells were neither selectively avoided nor ingested, but were consumed at rates comparable to those of ambient phytoplankton. The low density of toxic cells, and the fact that toxicity is not a perfect defense against grazers, probably contributed to the observed feeding behavior. Thus, in the southwestern Gulf of Maine, blooms of toxic Alexandrium spp. may experience significant grazing pressure from zooplankton. When copepods fed on toxic Alexandrium spp., toxins were accumulated in the tissues. A minor fraction of the calculated ingested toxin was retained in tissues or recovered in fecal pellets (≤5% each). The remaining toxin (∼90%) was lost to the environment, and literature evidence suggests that regurgitation and sloppy feeding contributed to this loss. The presence of alternate non-toxic prey did not affect retention efficiency. Toxin accumulation occurred in two phases; the first (exponential) phase corresponded closely to models of gut filling, while the second (linear) phase suggested toxin incorporation into tissues. Detoxification was rapid in most species of copepod tested