Zooplankton distribution in flowing waters and its implications for sampling: case studies in the River Meuse (Belgium) and the River Moselle (France, Luxembourg)

Zooplankton spatial distribution was studied on four occasions by transect sampling on the rivers Moselle and Meuse in July 1996. Sampling sites were selected for their variety in morphology. A pseudoreplicate-based sampling scheme was adopted that allowed small-scale longitudinal variations in dens ity to be tested along with transversal position and depth. In the Moselle, zooplankton were unevenly distributed transversally, an observation tentat ively linked with the complexity of the river channel and the influence of tributaries. In the Meuse, spatial heterogeneity was stronger, and depth al so played a key role. Its effect was different for rotifers and microcrusta ceans. The occurrence of zooplankton patches, similar to those commonly rep orted from lakes, was noted. The factors leading to the establishment of su ch distribution patterns, and their relevance to routine sampling, are brie fly discussed; a short list of recommendations for sampling in large rivets is proposed.