Sublittoral macrozoobenthic assemblages in the enclosed sediment-polluted Rhine-Meuse Delta; their relationship to environmental conditions

Composition, densities and biomass of sublittoral macrozoobenthos were investigated at 14 sites in the enclosed Rhine-Meuse Delta. The aim was to identify the main macrozoobenthic assemblages and to establish their relationship with environmental conditions, including sediment pollution. Most sites were dominated by the tubificidsLimnodrilus hoffmeisteri andL. claparedeanus. Four zoobenthic assemblages could be identified. They are related to different geomorphological phases of a basin filling up with fluvial sediment causing a longitudinal sediment gradient. The ’Rhine river’ assemblage was found in the Nieuwe Merwede, filled up in the 1970's, the ’soft mud’ assemblage in the Hollands Diep at sites with high sedimentation rates; the ’basin silt’ assemblage in the Amer, Hollands Diep and Haringvliet, at sites with lower sedimentation rates, and the ’basin shoal’ assemblage at former flats with low sedimentation rates. CANOCO was used to relate the TWINSPAN indicator species to a selection of environmental parameters. Sediment pH, water content, sand fraction, total polycyclic aromatic hydrocarbons (PAH) and current velocity explained 44% of the variance of the average abundance of the indicator taxa per site. Sediment firmness, indicated by sediment water content, and high ammonium concentrations probably were together responsible for the very low densities in the ’soft mud’ assemblage. Densities and biomasses in the other assemblages were low as well, except for theDreissena polymorpha habitat, where very high total zoobenthic biomass values (max. 55.8 g m⁻²) were observed. It is argued that sediment contamination has a considerable impact on macrozoobenthic densities and species composition in all assemblages, the ’basin shoal’ assemblage excepted.