Fine-scale vertical distributions of Mnemiopsis leidyi ctenophores: predation on copepods relative to stratification and hypoxia

Plankton concentrations near discontinuities in the water column (clines) are be lieved to be important for intensifying trophic interactions; however, evidence for increased feeding by predators at clines in situ is scarce. Here we demonstrate enhanced feeding near pycnoclines by a voracious planktivore, the ctenophore Mnemiopsis leidyi. To determine their feeding relative to stratification, we quantified temperature, salinity, dissolved oxygen concentration (DO), densities of ctenophores and copepods at 1 to 2 m depth intervals, and gut contents of ctenophores collected by depth layer at stations in a tributary and in the mainstem Chesapeake Bay during summer from 1999 to 2001. We tested the null hypotheses that patterns in the tributary and the bay were similar and that ctenophore vertical distributions and feeding were independent of the vertical distributions of the physical variables, stratification, and copepods. We rejected all null hypotheses. Ctenophores and copepods had peak densities below the pycnocline in the weakly stratified tributary, where DO was above 2 mg l−1 throughout the water column; by contrast, they were more concentrated above the strong pycnocline and near-anoxic waters at ~11 m in the bay. Predation on copepods by ctenophores was highest where both populations were concentrated. Our results illustrate the importance of stratification to planktonic trophic interactions for M. leidyi, which thrives in anthropogenically degraded waters and now is established throughout European seas, where it can negatively affect planktonic food webs and fisheries.