Spatial distribution of oligochaetes (Clitellata) in the tidal freshwater and brackish parts of the Schelde estuary (Belgium)

The benthic fauna of the Belgian part of the polluted Schelde estuary, called the Zeeschelde, was studied in September–October 1996 at 143 stations along 35 transects. This study is part of the OMES research program, funded by the Flemish Government, to build an ecosystem model of the Zeeschelde in order to help policy makers to decide upon the future of this unique estuarine system. Of all organisms retained on a 250 μm mesh-size, oligochaetes were co-dominant in the mesohaline part and the only dominant group in the tidal freshwater zone. The oligochaete fauna of the ß-mesohaline zone consisted of two brackish water tubificids, Heterochaeta costata and Tubificoides heterochaetus, and the euryhaline naidid Paranais litoralis. The freshwater species Limnodrilus hoffmeisteri penetrated as far as Boerenschans. Abundance in this zone is rather low with numbers increasing with exposure. The part of the oligohaline zone that coincides with the maximum turbidity zone is extremely poor in benthos, due to high physical, chemical and biological stress, with very low numbers of L. hoffmeisteri, Tubifex tubifex and P. litoralis. In the tidal freshwater section, mass populations of the tubificids L. hoffmeisteri and T. tubifex occur (peak densities of almost 3.106 ind m−2, maximum biomass: 25.7 g ADW m−2) with lower numbers of Limnodrilus claparedeianus, Limnodrilus udekemianus and Limnodrilus profundicola mainly in the part of the Zeeschelde close to Gent where better oxygen conditions are found. Oligochaetes are more abundant in finer sediments, resulting in a clear vertical gradient with low mean values in the deeper subtidal coarse sediments and highest numbers in the upper intertidal where fluid muds consolidate. In comparison with L. hoffmeisteri, T. tubifex was more abundant in the finest sediments. Vorticellidae are more common on tails of oligochaetes in the oxygen-enriched part (4–9%) than in the oxygen-deficient zone (< 0.5%). Regeneration rates, being considered as a suitable measure of predation pressure, increase with salinity and size of the worms and are highly specific. In comparison with other temperate estuarine systems, the Zeeschelde has impoverished communities with oligochaetes as the dominant taxa. Though a future improvement of the water quality will definitely result in a less productive but more varied benthic community in the freshwater tidal zone, oligochaetes will probably continue to play an important role due to sustained harsh conditions.