Effects of two different predators on zooplankton in vegetated patches of shallow eutrophic lakes: a mesocosm simulation

Macroinvertebrates and small fishes can both be important predators of zooplankton in littoral zones of shallow lakes. The aim of this study is to analyze how these predators may interact to affect zooplankton, algal biomass, and water turbidity. We performed a 15-day mesocosms experiment in 12 tanks (1300 l) simulating a littoral zone. A 2 x 2 factorial experiment was designed as follows: (1) Control without predators, (2) 7 nymphs of Anisoptera, (3) 2 visual zooplanktivorous fish and (4) 7 Anisoptera and 2 fishes. Results showed significant effects of fish on the abundance of Cladocera and its concentration among the macrophyte zone. On the other hand, we found no effects of Anisoptera on zooplankton abundance or distribution. No significant interaction between the predator effects was found on most response variables. Water turbidity increased only in the treatments with fish. Macrophytes offer a refuge for zooplankton to fish predation, but antagonist interactions appear when both predators were present, and the shelter effect of vegetated patches to the main herbivores against fish predation was less important when Anisoptera was present. The macroinvertebrate fauna of the littoral zone should be carefully considered in the restoration plans of shallow eutrophic lakes.