Reduction of zooplankton communities in small lake outlets in relation to abiotic and biotic factors

The aim of this study was to answer the following questions: (1) which environmental variables, biotic (fish predation) or abiotic factors, have a greater influence on the rate of zooplankton changes in lake outlet sections and (2) which plankters suffer the greatest reduction in the outlet section.  Samples were collected in two locations at each of 18 lakes. The first site was right at the outflow; the second site was 0.2 km downstream from the outflow. At downstream sites the percentage contribution of Rotifera in zooplankton species number and abundance was higher than in the outflow, whilst the percentage contribution of Cladocera and Copepoda (except nauplii) in the zooplankton species number and abundance was lower at downstream sites than in outlets. The most important variables affecting the rate of zooplankton reduction in the lake outlets are the biomass of cyprinids and hydrological parameters along the outlet, such as discharge, current velocity, and depth.  The most dramatic reductions involved daphnids, adult copepods, small cladocerans, and copepodites, while the reduction in the abundance of rotifers and nauplii was statistically insignificant.