Accumulation of red tide toxins in larger size fractions of zooplankton assemblages from Massachusetts Bay, USA

Phytoplankton toxins undergo trophic transport and accumulation in marine food webs, causing vectorial intoxication of upper-level consumers such as fishes, seabirds, and marine mammals. An entry point for phytoplankton toxins into these pelagic trophic pathways is frequently the herbivorous zooplankton. During the 1995 spring-summer red tide season in Massachusetts Bay, we examined accumulation of paralytic shellfish poisoning (PSP) toxins from the dinoflagellate Alexandrium spp. in various plankton size fractions (20-64, 6$-100, 100-200, 200-500, and >500 I-lm), and identified the relative composition of the zooplankton in these size fractions. Toxin levels were estimated by both high-performance liquid chromatography (HPLC) and a receptor-binding assay, the latter based on sample toxic potency. Although no PSP toxicity was detected in nearshore shellfish by routine monitoring programs using the mouse bioassay, positive responses were detected in zooplankton size fractions with the more sensitive HPLC and the receptor assay methods. The toxin signal was disproportionately concentrated in the larger zooplankton size fraction, frequently dominated by large copepods such as Calanus finmarchicus and Centropages typicus, which comprised only a small portion of total zooplankton abundance in quantitative samples obtained with 100 mu m mesh nets. By comparison, signal levels were low or undetectable in the smaller size fraction, which contained the overwhelmingly most-abundant zooplankters such as protists, copepod nauplii and copepodites and adults of small copepods such as Oithona similis, Paracalanus parvus, and Pseudocalanus spp. The larger toxin-accumulating copepods could provide a direct trophic linkage for vectorial intoxication of baleen whales that are known to feed upon such copepods.