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

- Predation by adult Mnemiopsis leidyi on mesozooplankton altered microplankton abundance, enhancing prey conditions for the larvae.
- A positive relationship between adult M. leidyi and microplankton (dinoflagellate and ciliate) abundance was identified.
- Increases in dinoflagellates and ciliates alongside adult M. leidyi blooms preceded an increase in ctenophore larvae.
- High larval abundances coincided with considerable declines in microzooplankton and a subsequent increase in nanoplankton.
- To our knowledge, this is the first study documenting in situ control of microzooplankton by larval M. leidyi.

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.

Planktonic food web of Great South Bay, NY. High abundances of adult Mnemiopsis leidyi reduce mesozooplankton predation pressure on microplanktonic dinoflagellates and ciliates, which subsequently increase in abundance. As a result, the feeding conditions and survival of larval ctenophores are improved, consequently reducing microplanktonic predation on nanoplankton.146