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, call ed the Zeeschelde, was studied in September-October 1996 at 143 stations al ong 35 transects. This study is part of the OMES research program, funded b y 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 estuar ine system. Of all organisms retained on a 250 mu m 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 con sisted of two brackish water tubificids, Heterochaeta costata and Tubificoi des heterochaetus, and the euryhaline naidid Paranais litoralis. The freshw ater species Limnodrilus hoffmeisteri penetrated as far as Boerenschans. Ab undance in this zone is rather low with numbers increasing with exposure. T he part of the oligohaline zone that coincides with the maximum turbidity z one is extremely poor in benthos, due to high physical, chemical and biolog ical stress, with very low numbers of L. hoffmeisteri, Tubifex tubifex and P. litoralis. In the tidal freshwater section, mass populations of the tubi ficids L. hoffmeisteri and T. tubifex occur (peak densities of almost 3.10( 6) ind m(-2), maximum biomass: 25.7 g ADW m(-2)) with lower numbers of Limn odrilus claparedeianus, Limnodrilus udekemianus and Limnodrilus profundicol a mainly in the part of the Zeeschelde close to Gent where better oxygen co nditions are found. Oligochaetes are more abundant in finer sediments, resu lting in a clear vertical gradient with low mean values in the deeper subti dal coarse sediments and highest numbers in the upper intertidal where flui d muds consolidate. In comparison with L. hoffmeisteri, T. tubifex was more abundant in the finest sediments. Vorticellidae are more common on tails o f oligochaetes in the oxygen-enriched part (4-9%) than in the oxygen-defici ent zone (< 0.5%). Regeneration rates, being considered as a suitable measu re 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 domina nt taxa. Though a future improvement of the water quality will definitely r esult in a less productive but more varied benthic community in the freshwa ter tidal zone, oligochaetes will probably continue to play an important ro le due to sustained harsh conditions.