Ctenophore-zooplankton-phytoplankton interactions in Narragansett Bay, Rhode Island, USA, during 1972-1977

Plankton dynamics at a station in lower Narragansett Bay, RIare compared for six summer and fall seasons, 1972–1977.In four of these years, initiation of the summer pulse of thectenophore Mnemiopsis leidyi was accompanied by a rapid declinein zooplankton abundance and a summer phytoplankton bloom. Terminationof the phytoplankton bloom coincided with depleted ctenophoreabundance and increased zooplankton biomass in two of the years.Yearly variations in the summer abundance of the diatom Skeletonemacostatum were positively related to the magnitude of the ctenophorepulse. The magnitude of ctenophore population was related tothe zooplankton biomass present at the start of the pulse. Theserelationships, the timing and magnitude of the plankton eventssuggest that M. leidyi regulated summer zooplankton and phytoplanktondynamics. Ctenophores may control phytoplankton blooms indirectlythrough their predation on herbivorous zooplankton and directlyby the nutrient excretion accompanying such grazing. This evidencethat a planktonic carnivore two trophic steps removed from thephytoplankton regulates the latter's dynamics in NarragansettBay is analogous to reported regulation of benthic algal (kelp)dynamics by the sea otter, lobster and various crabs throughtheir predation on herbivorous sea urchins. The factors responsiblefor the seasonal decrease in ctenophores remain unresolved;ctenophore predators on Mnemiopsis are absent in NarragansettBay. Infection by the vermiform larval anemone, Edwardsia lineata,grazing by the butterfish, Peprilus triacanthus, and changesin food availability, temperature and salinity likewise do notexplain this disappearance.