Nematode assemblages from subtidal sandbanks in the Southern Bight of the North Sea: effect of small sedimentological differences

Nematode assemblages from four subtidal sandbanks belonging to different sandbank systems on the Belgian Continental Shelf were investigated both in spring and fall. The assemblages were characterised by different species composition patterns on the different sandbanks. This is in contrast to results of earlier studies which showed that neither meiobenthic nor macrobenthic taxa differed among these sandbanks. Although the sediments on these sandbanks could all be classified as medium sands, the use of Multiple Discriminant Analysis (MDA) suggested that median grain size and the proportions of median sand and very fine sand were the variables explaining the difference in nematode community composition. These findings emphasise the strong relationship between the relative abundance of nematode species and sediment composition. The influence of sand extraction on these sandbanks resulted in coarsening of the sediment, which had a direct effect on the nematode species composition. Diversity was not affected, indicating that nematodes inhabiting highly dynamic environments are well adapted to physical disturbance. The diversity at sandbanks is not necessarily very different from the surrounding areas, since in more offshore parts of the Belgian Continental Shelf, clean and rather coarse sands prevail and the differences in sediment composition are not sufficient to induce large differences in diversity.