The subtidal macrobenthos in the mesohaline part of the Schelde Estuary (Belgium): influenced by man?

The macrobenthos of the subtidal, mesohaline zone of the Schelde Estuary (Belgium) was sampled in October 1996 and 1997 at 54 and 73 sampling locations respectively. Sediments ranged fr om silty to very coarse, with the dominant sediment type being silt (33% of all locations). Of the 35 macrobenthic species observed, only seven species occurred in more than 20% of the samples. The polychaete Heteromastus filiformis and Oligochaeta were most common. Multivariate techniques revealed three distinct communities, linked mainly with sedimentological factors: (1) a species-poor (nine species) community with a dominance of the amphipod Bathyporeia pilosa, a low mean abundance and biomass (86 ind/m², 0.0189 g ash-free dry weight (AFDW)/m²), and a mean median grain size of 215 +/- 19 µm (fine sand); (2) a species-rich (22) community, with the small polychaete Polydora ligerica as indicator species, a relatively high mean abundance and biomass (2298 ind/m², 1.395 g AFDW/m², oysters excluded), a mean median grain size of 133 +/- 41 µm, and also the occurrence of sediments with hard substrates being characteristic for this community; (3) a community with an intermediate species richness (12), abundance and biomass (248 ind/m², 0.249 g AFDW/m²), with H. filiformis and Oligochaeta as indicator species, and a median grain size of 76 +/- 9 µm. In the study area several typical brackish water species were observed (e.g. Polydora ligerica, Corophium lacustre, Gammarus salinus). Mean total abundance and biomass were very low, and the benthic communities appeared to be under stress, with a dominance of mainly small, subsurface deposit and surface deposit feeding opportunistic species. This is probably a combined effect of both natural physical and human-induced disturbance. Only sediments with hard substrates (e.g. rocks) seems to favour species richness, providing a shelter against physical disturbance