The mesozooplankton community of the Belgian shelf (North Sea)

This manuscript presents themesozooplankton community structure and its spatial and temporal variabilities in the Belgian part of the North Sea (BPNS), a first thorough study on this topic in nearly 40years.Monthly sampling campaigns at ten stations in the BPNS in 2009 and 2010 yielded a total of 137 mesozooplankton taxa (46 holoplanktonic, 50 meroplanktonic and 41 tychoplanktonic), of which nine species had never been reported in the area. Smaller neritic copepods, especially Temora longicornis and Acartia clausi, were present in all samples and dominated zooplankton densities (66%), together with the appendicularian Oikopleura dioica (10%). They were joined by high numbers of meroplanktonic echinoderm larvae (9%) in spring and summer. Based on
diversity alone, the mesozooplankton could be typified as one neritic zooplankton community, due to the ubiquitous presence in time and space of the dominant copepods. Yet, these neritic species were often joined
by low numbers of oceanic species that are occasionally imported with the inflow of Atlantic oceanic water in the BPNS. Based on a combination of abundance and diversity, our results indicate distinct seasonal and spatial distribution patterns in the mesozooplankton. Months with highest average densities were May, June and July, lowest densities were noted in December and January. Only limited long-term zooplankton data are available for the BPNS from the Continuous Plankton Recorder surveys or the long-termmonitoring stations in the vicinity
of our research area. However, our data suggest that nowadays zooplankton species appear earlier in the BPNS, comparable with other areas in the North Sea. Densities varied between 150 and 15,000 ind.m−3, and averaged highest at midshore stations, then nearshore and offshore. This is partially comparable with the spatial patterns recorded for other ecosystem components, such as demersal fish, epibenthos and macrobenthos, of which densities peak in a stretch almost parallel to but some miles away from the coastline in the BPNS.