Plankton dynamics in a dead arm of the River Waal: a comparison with the main channel

Abstract

It has been postulated that areas of reduced flow along rivers, either within the river channel or in the form of backwaters, exert a considerable influence on plankton dynamics in rivers. To test this hypothesis, a survey of plankton distribution in a dead arm along the River Waal, a branch of the River Rhine, was made. The euphotic fraction of the water column varied along the transect. Phytoplankton species composition changed from diatom-dominated populations in the river to dominance of green algae near the rear end of the dead arm. Green algae were dominated by Pandorina morum. The diatoms were dominated by typically riverine species, like Cyclotella menegh-iniana, Skeletonema potamos and Skeletonema subsalsum . The total zooplankton biovolume at the rear end was >100 times higher than in the river. In the river, rotifers and crustaceans co-dominated, whereas to the rear end the crustaceans became dominant. Diaphanosoma brachyurum was the most abundant zooplankter. The dynamic behaviour of both phytoplankton and zooplankton suggests the presence of an intense grazing network in the dead arm. There were also some indications of a rapid transfer of biomass to higher consumer levels, such as fish. The large difference in species composition between the dead arm and the river itself does not suggest a major influence by backwaters along the River Rhine. This may be due to its channelized nature. The situation can be different in rivers with a more natural morphology.