Seasonal and longitudinal aspects of microcrustacean (Cladocera, Copepoda) dynamics in a lowland river

Abstract

Crustacean zooplankton was sampled extensively from the River Meuse between 1993 and 1998. The spatio-temporal aspects of their dynamics were examined in relation to the primary forcing factor: hydrology. A time-series obtained at a fixed sampling station revealed a strong dependence of development on discharge patterns, as expected, despite large interannual variability in the recorded maximal densities. Benthic cladocerans were most sensitive to rises in discharge. On the longitudinal plane, much higher densities were recorded in downstream reaches of the river, over distances too short for the necessary number of generations to develop at the generally accepted rates of growth. Short-lived peaks in abundance of large planktonic cladocerans were also recorded downstream, at a well-defined time of the year. These observations suggest that hydrology is not always accountable for all the variations observed, and that the combination of supplementary zooplankton inputs, internal production and trophic effects should also be invoked. In the case of copepods, for the most part represented by cyclopoids, the hypothesis of resident populations is proposed to account for seemingly unrealistic rises in density.