Nematode communities from the North Sea: environmental controls on speciesdiversity and vertical distribution within the sediment

Nematode assemblages were sampled seasonally at three subtidal stations alo ng the Belgian coast. The stations were characterized by muddy sediments (s tation 115), fine sand (station 702) and fine to coarse sand (station 790). The forces structuring vertical distribution were investigated by evaluati ng abundance, species composition, diversity and trophic composition, and r elating these to sediment composition, redox state and food sources. The nematode assemblages at the two finer grained stations (115, 702) were dominated by Daptonema tenuispiculum and Sabatieria punctata. For both spec ies, the vertical distribution in the sediment seemed not dependent on the redoxchemistry, as former believed for S. punctata, but primarily influence d by food availability. This feature could also be recognized for Ixonema s ordidum and Viscosia langrunensis, the most abundant nematodes at the coars e sandy station (790). In general, nematode diversity was regulated primarily by sediment granulom etry. Coarser sediments (station 790) yielded more diverse communities comp ared to the fine sediments (station 115, 702), however seasonal fluctuation s and variations with depth into the sediment were not obvious. At the silt y stations, when the sediment column was more oxidized in March, overall di versity was higher and showed a positive relationship to the mud content wh ich varied with depth into the sediment. This positive relation is probably explained by an enhanced deposition of organic matter associated with the accumulation of fine particles near the river-mouths. Furthermore, the high er abundance, the lower diversity and the higher dominance found at the two silty stations of the eastern and the western part of the Belgian coast, p ointed to a stressed, organically enriched environment. The results demonstrate that controls on nematode community structure are c omplex and that information at both species and community level are require d to properly evaluate the effects of natural and anthropogenic impacts.