Top-down control of mesozooplankton by adult Mnemiopsis leidyi influences microplankton abundance and composition enhancing prey conditions for larval ctenophores

The ctenophore Mnemiopsis leidyi is a gelatinous predator capable of exerting strong regulatory control on the zooplankton community. Mnemiopsis populations are comprised of lobate adults and cydippid larvae. Since the larvae depend on microplankton for prey, its availability may determine the magnitude of larval survivorship and growth, and their subsequent recruitment into mesozooplankton-feeding adults. Ctenophore population data were used alongside mesozooplankton and microplankton abundances to interpret predatory impacts of M. leidyi in a Long Island, New York estuary over two years. Field data suggested significant top-down control of mesozooplankton and microplankton during peak abundances of adult and larval ctenophores, respectively. Abundances of dinoflagellates and ciliates declined by 45-56% and 83-97%, respectively, during highest larval abundances in 2008 and 2009. Furthermore, the dramatic reduction of mesozooplankton by adult M. leidyi resulted in a cascading effect on microplankton. A relationship between high adult M. leidyi/low mesozooplankton with high microplankton abundances was identified, and preceded an increase in ctenophore larvae. These data suggest that blooms of M. leidyi result in a direct feedback system, wherein intense feeding activity by adults on mesozooplankton releases certain microplanktonic taxa from predation pressure, enhancing prey conditions for larval ctenophores.