Diel changes in the horizontal distribution of zooplankton in the littoral zone of two shallow eutrophic lakes

Abstract

Diel horizontal migration of zooplankton was studied in the littoral zone of two shallow Danish lakes. In Lake Vaeng, in which emergent macrophytes and tree roots were present in the littoral zone, sampling was undertaken at five locations at 0.5 m and 5 m from the shore. The largest diel changes seen in the mean density of Daphnia hyalina/galeata SARS were a decrease from a daytime maximum of 1,766 indiv 1 -1 to a night-time minimum of 638 indiv 1 -1 at the stations 0.5 m from the shore, and an increase from 3 indiv 1 -1 to 162 indiv 1 -1 at the stations 5 m from the shore. In Lake Ring, in which both macrophyte beds (Potamogeton pectinatus L.) and open areas are present in the littoral zone, sampling was undertaken at 24 locations along a transect running parallel to the shore. The mean density of Daphnia magna STRAUS within the macrophyte beds was up to 20-fold higher during daytime than during night-time (40 indiv 1 -1 and 2 indiv 1 -1 , respectively), whereas no general changes were observed in the open areas. The greatest diel changes were found near the edge of the macrophytes. The density of Daphnia hyalina/galeata in the macrophyte beds was also less during night-time than daytime, but the difference was not as marked. The mean density of cyclopoids was 2-3 fold higher in the macrophyte beds than in the open areas, but no diel changes in density were observed. We interpret these findings as indicating that Daphnia magna and Daphnia hyalina/galeata undertake diel horizontal migration, and that macrophytes have a positive impact on large filter-feeding zooplankton in that they expand the area serving as a potential refuge from fish predation.