Role of eutrophication in structuring planktonic communities in the presence of the ctenophore Mnemiopsis leidyi

Increasing evidence implicates anthropogenic activities with recently documented shifts in the abundance and seasonal distribution of gelatinous zooplankton in coastal waters. The ctenophore Mnemiopsis leidyi occurs in mid-Atlantic estuaries in the USA where seasonal blooms occur earlier and in greater magnitude than those studied decades ago. Large densities of adult M. leidyi exert significant predation pressure on mesozooplankton, potentially influencing microplankton abundance and composition. Field-based mesocosm experiments were conducted to examine the individual and interactive roles of ctenophore predation and nutrient loading on the microplankton community using historic and recent abundances of M. leidyi in Great South Bay, New York, USA. High (recent) abundances of M. leidyi exposed to eutrophic conditions influenced plankton community structure in a way that was distinctly different from when the processes occurred separately or under low (historic) abundances. Microplanktonic ciliates exhibited an order of magnitude increase in tanks receiving either nutrient or ctenophore amendments, but increased by 2 orders of magnitude in treatments receiving both ctenophore and nutrient additions. Furthermore, ctenophores recovered from nutrient treatments produced nearly 3 times as many eggs than those reclaimed from ctenophore-only treatments, suggesting that nutrient enrichment enhances fecundity of M. leidyi. Since ciliates are an important prey item for developing M. leidyi, the combined bottom-up and top-down influences of eutrophication and ctenophore predation, respectively, on microplankton may help explain recently documented shifts in the population dynamics of M. leidyi in mid-Atlantic estuaries.