Indirect enhancement of large zooplankton by consumption of predacious macroinvertebrates by littoral fish

Abstract

Food webs in the littoral zones of shallow lakes are inherently complex. Submerged macrophytes are considered refuge areas although they host potential pred- ators for zooplankton. During spring : summer of 2000/2001, we carried out two meso- cosm experiments in Los Padres Lake, a shallow macrophyte dominated lake of the Argentine Pampa plain. We investigated the effect of littoral fish predation on zoo- plankton and the role of submerged macrophytes and benthic macroinvertebrates in the zooplankton-fish relationship. Treatments differed in macrophyte cover (0-65 % PVI) and fish presence. In both experiments, we determined zooplankton abundance and body size distribution, as well as macroinvertebrate abundance. The addition of the small littoral fish Astyanax eigenmanniorum (Characidae, Tetragonopterinae) to the enclosures had either a positive or a negative effect on large zooplankton, especially on calanoid copepods, depending on macroinvertebrate densities and availability. Fish did not impact on small size Cladocera. Instead, littoral fish preferred macroinverte- brates to zooplankton, and thus, the impact of fish predation on plankton community depended largely on the macroinvertebrate abundance. Therefore, our results suggest that macrophyte-fish complex could enhance or depress zooplankton abundance and that the effect is related with predatory macroinvertebrates associated with vegetation.