In the Western Baltic, the invasive ctenophore Mnemiopsis leidyi was recorded for the first time in autumn 2006. An eastward propagation of the ctenophore into the central Baltic, and thus into important spawning grounds of major Baltic fish stocks, was observed in 2007. The focus of the present study was to investigate the seasonal phenology of this introduced species and whether it is able to form a self-sustaining population in this area. Therefore, the variability of temporal and spatial distribution of M. leidyi in the Bornholm Basin was analyzed over the first 4 years following the invasion and related to ambient hydrographic parameters. Results show a clear seasonal pattern. In contrast to the majority of other native and exotic habitats, the seasonal phenology showed highest abundances in spring and autumn months and only sporadic or even no appearance during summer. Vertical distribution was mostly confined to water layers below the permanent halocline and significantly influenced by ambient temperature. Our results indicate that there is no self-sustaining population of M. leidyi in the central Baltic Sea. Instead, the species is most likely re-introduced into the Bornholm Basin every year via lateral advection from source populations in the Western Baltic. These findings are important not only to further assess the potential impact of M. leidyi on the pelagic ecosystem of the central Baltic Sea, but also for a better understanding of the mechanisms of its invasion into other marine areas.
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