Lodewijk van Walraven

First record of Caryophyllia smithii in the central southern North Sea: artificial reefs affect range extensions of sessile benthic species

In the North Sea, observations of the solitary coral Caryophyllia smithii are mostly limited to the north-east coast of the United Kingdom including the Shetland and Orkney islands. To date, C. smithii has not been reported from far offshore locations in the North Sea south of 57.78N. Distribution of this species appears limited by the restricted availability of natural hard substrata in the central southern North Sea. There are, however, many artificial hard substrata in this area in the form of shipwrecks and offshore oil and gas installations. These may provide stepping stones for C. smithii to expand its distribution. Here we report the first sighting of C. smithii in the central southern North Sea on an unidentified wreck on the Dogger Bank. This is the first offshore observation of any hard coral in the central southern North Sea.

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. δ15N 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 δ13C and δ15N 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 δ15N 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.