Spatial patterns of planktonic microcrustaceans in a small shallow lake

The horizontal distribution of planktonic microcrustaceans was studied at two different spatial scales in a small shallow lake (Creteil Lake, Paris suburb, France) during the summer. At all scales of sampling, the distribution was patchy. However, the variations between sampling units were smallest when considering the smallest sampling grid (30 sampling units in a 0.5 ha area). Positive correlations between densities of most taxa were observed at this scale, indicating that a common set of factors could influence zooplankton patchiness. Physical factors, such as wind-induced currents, strongly influenced the lakewide distribution of zooplankton (43 sampling units in a 42 ha area). The horizontal pattern of each taxon appeared to be linked to its ability to form vertical aggregation. Moreover, offshore-nearshore differences in fecundity of several taxa suggest the action of biotic factors in the horizontal distribution of zooplankton. A higher predation risk in the littoral neighbourhood seems to be the more probable proximate--or ultimate--factor which could explain this pattern. In this small and shallow lake, most factors that can affect distribution of zooplankton are temporally variable and unpredictable. In the absence of automatic and efficient measuring techniques, the realization of a small number of integrated samples at the level of the whole ecosystem could be a simple and suitable solution to obtain statistically reliable data on zooplankton demography.