Distribution patterns, population-dynamics and role of Oligochaeta in the Zeeschelde tidal ecosystem

This study on bentic invertebrates of the Zeeschelde (the Belgian tidal part of the Schelde) is part of the OMES (Onderzoek Milieu Effecten Sigmaplan) research program, funded by the Flemish Government and coordinated by the Institute of Nature Conservation. This project aims at developping an ecosystem model that will guide policy-makers in the future management of the Zeeschelde. The benthic fauna of the Zeeschelde, is dominated by oligochaetes. Particularly in the tidal freshwater part where sedimentation of fine sediments provide optimal food supply, they form mass populations. An assemblage of cosmopolitan very eurytopic species (Limnodrilus hoffmeisteri, L.claparedeianus, L.udekemianus, L.profundicola, Tubifex tubifex) can be found here in peak densities of nearly 3.10⁶ ind.m^-2 or 25,7 9 AFDW.m^-2 (equivalent to > 1 kg wet weight.m^-2!) with increasing numbers in finer, more exposed sediments. Low oxygen availability and high physical stress, prevent other species from entering the tidal freshwater zone. The B-mesohalinicum (Antwerpen to Belgian-Dutch border) is more diverse with several species of polychaetes, molluscs and crustaceans characteristic for the brackish reaches of European estuaries. Dominant oligochaetes here are Heterochaeta costata, Tubificoides heterochaetus and Paranais litoralis. Maximal densities are much smaller than in the freshwater zone (up to 200.000 ind.m^-2) and higher on the intertidal flats than in the coarser sediments of the gullies and shallow subtidal strata. A very poor benthic community is found in the oligohaline zone (Rupel mouth to Antwerpen) due to a very high physical, chemical and biological stress. Temporal patterns of oligochaete abundance show low values in winter, growing populations from May onwards, peaking in September- October and crashing between November and January. It is suggested that more than 10.000 ducks take advantage of this highly energetic abundant food resource in the Zeeschelde, causing the population crash. The potential role of Oligochaeta in the Zeeschelde is further discussed.