Efficiency of top-down control depends on nutrient concentration in a Mediterranean shallow lake: A mesocosm study
Saúl Blanco A B , Margarita Fernández-Aláez A and Eloy Bécares AA Department of Biodiversity and Environmental Management, University of León, E-24071 León, Spain.
B Corresponding author. Email: sblal@unileon.es
Marine and Freshwater Research 59(10) 922-930 https://doi.org/10.1071/MF07212
Submitted: 7 November 2007 Accepted: 5 August 2008 Published: 27 October 2008
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.
Additional keywords: detritivory, diet content, eutrophication, food web, uncoupling, zooplanktivory.
Acknowledgements
This work was supported by the EU project SWALE (Contract ENV4-CT97–0420). The authors are grateful to the members of the project team in León. S. Romo and two anonymous referees are thanked for their valuable comments on the manuscript. P. Budy kindly revised the English draft. Sampling procedures and fish handling were done in accordance with Spanish regulations.
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