Combined effects of fish and macroinvertebrate predation on zooplankton in a littoral mesocosm experiment

Whether macrophytes offer an effective refuge for zooplankton in all shallow lakes is subject to debate. To explore potential constraints between different predator threats and the related habitat choice by zooplankton, we conducted a mesocosm experiment in 12 large-sized pools mimicking the nearshore environment with part of its length being covered by submersed macrophytes (Egeria densa) and holding a mixed zooplankton community. Four treatments were used: (i) young zooplanktivorous fish (3 silverside, Odontesthes bonariensis) in the “open-water” zone; (ii) macroinvertebrate predator (31 grass shrimp, Palaemonetes argentinus) in the vegetated zone; (iii) both, fish in the open-water and shrimp in the vegetated zones; and (iv) control with no predators. Our results show specific effects of each predator on the abundance, composition, and size of cladocerans. Regarding distribution, in control and shrimp mesocosms, no differences were found between the two zones, while cladocerans were clearly more abundant in the vegetated side in the presence of fish. When both fish and shrimp were present, cladocerans preferred the vegetated zone too, but in a smaller proportion, and their abundance was less. The presence of predatory macroinvertebrates in vegetated littoral zone reduces the refuge value of this habitat, at least for cladocerans.