Trophic accumulation of PSP toxins in zooplankton during Alexandrium fundyense blooms in Casco Bay, Gulf of Maine, April-June 1998. II. Zooplankton abundance and size-fractionated community composition

During spring blooms of the toxic dinoflagellate Alexandrium fundyense in Casco Bay, Maine in 1998, we investigated vectorial intoxication of various zooplankton size fractions with PSP toxins, including zooplankton community composition from quantitative zooplankton samples (>102 mu m), as well as zooplankton composition in relation to toxin levels in various size fractions (20-64, 64-100, 100-200, 200-500, >500 mu m). Zooplankton abundance in 102 mu m mesh samples was low (most values-10,000 animals m super(-3)) from early April through early May, but increased to maxima in mid-June (cruise mean=121,500 animals m super(-3)). Quantitative zooplankton samples (>102 mu m) were dominated by copepod nauplii, and Oithona similis copepodites and adults at most locations except for those furthest inshore. At these inshore locations, Acartia hudsonica copepodites and adults were usually dominant. Larger copepods such as Calanus finmarchicus, Centropages typicus, and Pseudocalanus spp. were found primarily offshore, and at much lower abundances than O. similis. Rotifers, mainly present from late April to late May, were most abundant inshore. The marine cladoceran Evadne nordmani was sporadically abundant, particularly in mid-June. Microplankton in 20-64 mu m size fractions was generally dominated by A. fundyense, non-toxic dinoflagellates, and tintinnids. Microplankton in 64-100 mu m size fractions was generally dominated by larger non-toxic dinoflagellates, tintinnids, aloricate ciliates, and copepod nauplii, and in early May, rotifers. Some samples (23%) in the 64-100 mu m size fractions contained abundant cells of A. fundyense, presumably due to sieve clogging, but most did not contain A. fundyense cells. This suggests that PSP toxin levels in those samples were due to vectorial intoxication of microzooplankters such as heterotrophic dinoflagellates, tintinnids, aloricate ciliates, rotifers, and copepod nauplii via feeding on A. fundyense cells. Dominant taxa in zooplankton fractions varied in size. Samples in the 100-200 mu m size fraction were overwhelmingly dominated in most cases by copepod nauplii and small copepodites of O. similis, and during late May, rotifers. Samples in the 200-500 mu m size fraction contained fewer copepod nauplii, but progressively more copepodites and adults of O. similis, particularly at offshore locations. At the most inshore stations, copepodites and adults of A. hudsonica were usual dominants. There were few copepod nauplii or