Abstract
Marine invasions are of increasing concern for biodiversity conservation worldwide. Gelatinous macrozooplankton contain members, which have become globally invasive, for example the ctenophore Mnemiopsis leidyi or the hydromedusae Blackfordia virginica. B. virginica is characterised by a large salinity tolerance, with a brackish-water habitat preference, and by a metagenic life history strategy with an alternation between sexually reproducing planktonic medusae and asexually reproducing benthic polyps to complete the life cycle. In this study we analysed 8 years of ichthyoplankton survey data (2010–2017) from the Kiel Canal and 14 ichthyoplankton summer surveys in the central Baltic Sea (2008–2017). We report the first presence of B. virginica in northern Europe, namely from the southwestern Baltic Sea and the Kiel Canal. In the Kiel Canal, B. virginica was first sporadically sighted in 2014 and 2015 and has developed persistent populations since summer 2016. Changes in size-frequency distributions during summer 2016 indicate active recruitment in the Kiel Canal at salinities between 7 and 13 and temperatures > 14 °C. Close vicinity to and direct connection with the southwestern Baltic Sea, where B. virginica was observed during 2017, indicate that the Baltic Sea and other brackish-water habitats of Northern Europe are at risk for colonisation of this non-indigenous species. Our results highlight that monitoring activities should consider gelatinous macrozooplankton for standard assessments to allow for the detection of non-indigenous species at an early stage of their colonisation.
Highlights
Global change has wide ranging consequences for the productivity and functioning of the worlds’ oceans
In this study we describe the first record of Blackfordia virginica in the Kiel Canal and Kiel Fjord in the southwestern Baltic Sea
Sequence analyses with four primers of three different marker genes showed that the general 16S sequences of sampled individuals were 100% identical to Blackfordia virginica (Mayer 1910) sequences retrieved from GenBank
Summary
Global change has wide ranging consequences for the productivity and functioning of the worlds’ oceans. It has been documented that the rate at which new species are recorded outside their natural habitat boundaries is steadily increasing, with no signs of saturation [4]. Even though it is only a small fraction of the originally arriving non-indigenous species. The Baltic Sea has a very high proportion of non-indigenous species compared to the total number of species [8], in that respect ranking among the top 10 affected ecosystems worldwide [9] It is characterised by a very steep salinity gradient ranging from 2 in the north east (Bothnian Bay) to 31 at the transition zone to the North Sea in the west [10]. This is especially so as the 98 km long Kiel Canal, which connects the North Sea to the SW Baltic Sea, is the busiest
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