PLANKTON DYNAMICS IN A DEAD ARM OF THE RIVER WAAL - A COMPARISON WITHTHE MAIN CHANNEL

It has been postulated that areas of reduced flow along rivers, either within the river channel or in the form of backwaters, exert a consid erable influence on plankton dynamics in rivers. To test this hypothes is, a survey of plankton distribution in a dead arm along the River Wa al, a branch of the River Rhine, was made. The euphotic fraction of th e water column varied along the transect. Phytoplankton species compos ition changed from diatom-dominated populations in the river to domina nce 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 meneghiniana, Skeletonema potamos a nd Skeletonema subsalsum. The total zooplankton biovolume at the rear end was >100 times higher than in the river. In the river, rotifers an d crustaceans co-dominated, whereas to the rear end the crustaceans be came dominant. Diaphanosoma brachyurum was the most abundant zooplankt er. The dynamic behaviour of both phytoplankton and zooplankton sugges ts the presence of an intense grazing network in the dead arm. There w ere also some indications of a rapid transfer of biomass to higher con sumer levels, such as fish. The large difference in species compositio n between the dead arm and the river itself does not suggest a major i nfluence by backwaters along the River Rhine. This may be due to its c hannelized nature. The situation can be different in rivers with a mor e natural morphology.