Interannual differences in microplankton drive changes in the feeding and fecundity of Mnemiopsis leidyi in a Long Island estuary

Abstract

The ctenophore Mnemiopsis leidyi forms seasonal population blooms, which exhibit interannual variation in abundance. Ctenophore egg production and gut contents were examined alongside plankton community structure to identify factors influencing recruitment of M. leidyi in Great South Bay, NY. Ctenophores contained three times as many prey items and produced twice as many eggs in 2008 during a brown tide (Aureococcus anophagefferens) than in 2009, a year without a brown tide. Gut contents demonstrated significant dependence on the copepod Acartia tonsa, which is known to switch from ambush to suspension feeding when flagellates are abundant. Microflagellate abundance was significantly greater in 2008 and corresponded positively with A. anophagefferens, potentially increasing the encounter rates between M. leidyi and A. tonsa by promoting suspension feeding in the latter. Despite the enhanced fecundity, however, ctenophore abundances were five times lower in 2008 and a mismatch was identified between optimum egg production by adults and sufficient microplanktonic prey (dinoflagellates and ciliates) abundance for their larvae, compared to 2009 when the two coincided. We propose that increased microflagellate abundance during the brown tide provided a benefit to ctenophore fecundity by enhancing their encounter rates with A. tonsa, but ultimately larval recruitment was limited by insufficient food availability.