Efficiency of top-down control depends on nutrient concentration in a Mediterranean shallow lake: A mesocosm study

Abstract

In shallow lakes, the influence of top-down control is considered to be less important under a Mediterranean bioclimate, although little experimental data support this hypothesis. The present study aimed to experimentally assess the importance of top-down forces in the trophic status of Mediterranean shallow lakes. A bifactorial mesocosm experiment was carried out in a shallow lake using a range of nutrient concentrations and fish population densities. The results demonstrated an expected increase in turbidity levels as nutrients and fish density increased, as a result of both sediment resuspension and chlorophyll a concentrations. Nevertheless, at higher nutrient additions, turbidity decreased in the mesocosms in response to cladoceran proliferation because the fishes were unable to control zooplankton growth under such conditions. Fish diet content analysis showed a generalist diet with an important contribution of non-planktonic components; the proportion of zooplankton in the diet contents increased with nutrient additions. Fishes preferred cladocerans over copepods and rotifers, these latter taxa dominated the resulting zooplankton community. Some fish-added mesocosms retained high transparency levels, most likely because of physiological changes in response to high ammonium concentrations. In summary, the dietary spectrum of Chondrostoma arcasii [Steindachner], despite some electivity patterns, depends largely on available resources. The trophic ecology of cyprinids, acting both through direct and indirect effects on food webs, is a determinant factor explaining, in part, stable high turbidity levels in many shallow lakes.