Gelatinous zooplankton in the Belgian part of the North Sea and the adjacent Schelde estuary: spatio-temporal distribution patterns and population dynamics

Global ocean ecosystems are thought to be heading towards a gelatinous future, a perception that is strengthened by an increased reporting of problems caused by jellyfish. However, gelatinous zooplankton has been largely understudied in most ecosystems worldwide, and the absence of quantitative long-term abundance data impedes drawing conclusions on the potential rise of these organisms. This study presents results from monthly samples taken in 2011-2012 at six locations in the Belgian part of the North Sea and the adjacent Schelde estuary. Three Scyphozoa, three Ctenophora and 27 Hydrozoa taxa were identified. Densities of gelatinous zooplankton never outnumbered the non-gelatinous zooplankton in our samples. Average gelatinous zooplankton densities reached up to 18 ind.m-3 near the coast and gradually declined towards the open sea (60 km offshore). In the brackish Schelde estuary, average densities remained below 4 ind.m-3. Gelatinous zooplankton reached highest densities in autumn throughout the study area. We conclude that the observed distribution of gelatinous zooplankton is driven by a limited number of environmental variables (temperature related to season and salinity related to location). Three non-indigenous gelatinous species were found in the area: Nemopsis bachei, Lovenella assimilis and Mnemiopsis leidyi. The peak densities of the latter did not yet reach worrying densities but data from this and other studies indicate that the Dutch estuaries and Belgian ports provide optimal conditions and potential for a year round presence and thus reproduction of M. leidyi. Further regular monitoring is needed to answer the gelatinous future paradigm for the Southern North Sea.