Zooplankton grazing impacts on Alexandrium spp. in the nearshore environment of the Gulf of Maine

We collected samples at weekly intervals from several stations in coastal waters of the Gulf of Maine during the spring of 1998 and 1999 for zooplankton and phytoplankton abundance, biomass, species composition, and toxin content. In addition, grazing rates of zooplankton were determined using natural water from selected stations. During 1998, there was a moderate bloom of the paralytic shellfish poison (PSP) producing dinoflagellates Alexandrium spp. (3000 cells/L), while in 1999 concentrations were very low throughout the study. In 1998, potential zooplankton grazing-impacts on Alexandrium spp. increased from 0 to 0.8 day−1 in concert with the vernal increase in zooplankton biomass and appeared to contribute to the bloomʹs demise. During the 1998 bloom, PSP toxin levels in zooplankton tissues appeared to be sufficient to pose risks to higher trophic levels, such as fishes and marine mammals. Our findings suggest that zooplankton grazing can be an important source of mortality for harmful algal bloom species such as Alexandrium spp.