Temporally varying isotopic niche overlap of the invasive ctenophore Mnemiopsis leidyi with other zooplanktivores in the western Dutch Wadden Sea

The invasive ctenophore Mnemiopsis leidyi can be a major predator of zooplankton in areas where it has been introduced. In this study, the possible competition of M. leidyi with native macroplankton and nekton in the western Dutch Wadden Sea was investigated in March-August, 2011 by determining and comparing isotopic niches of zooplanktivores. Stable carbon and nitrogen isotope signatures were determined from tissue samples of fish, scyphozoa, hydromedusa, ctenophores, crustaceans and cephalopods.{delta} 15N of M. leidyi was positively related to ctenophore size, suggesting that small ctenophores occupied a lower trophic level than large ones. A cluster analysis showed that in the spring and early summer period, when M. leidyi densities are low, average{delta} 13C and{delta} 15N ratios of the invasive M. leidyi were similar to those of most other gelatinous zooplankton and pelagic fish species sampled. At the beginning of the bloom period in August there was no overlap in isotopic niche of M. leidyi with that of any other pelagic zooplanktivore. During this month the population consisted mainly of larvae and juveniles with lower{delta} 15N ratios. At present, M. leidyi appears not to be a significant competitor for other gelatinous zooplankton and fish species because the period of high diet overlap with other consumers was also the period in which M. leidyi was least abundant.