The role of fish communities in the eutrophication of Shallow Lakes

Abstract

The progressive anthropogenic eutrophication, the uncontrolled introduction of exotic fish species, and the lack of conservation and management strategies are highlighted as the main causes explaining the long-term changes in water biological quality in freshwater ponds and their surrounding aquatic ecosystems worldwide. Experimental evidence shows that fish direct and indirect effects on lake food web lead to algal growths and increased turbidity and nutrients concentration in fish-added treatments. Consequently, submerged macrophytes and epiphytic algae usually decline at increasing nutrient levels. Additionally, many freshwater fishes exhibit a generalist diet with an important contribution of non-planktonic components, and ontogenic shift towards bentho-detritivory has been observed in several species. Field studies on the feeding ecology of both endemic and introduced taxa show that demographic changes and facultative predation influence food webs in freshwater ponds by both direct predation and indirect effects (sediment bioturbation and excretion), however, cascading effects caused by introduced fish species may be more relevant for the food webs in certain bioclimates, e.g. Mediterranean shallow ponds and wetlands, where almost no piscivorous species are recorded and, in general, omnivorous species dominate independently of lake trophic state. Thus, the role of direct and indirect effects of fishes and the importance of their dietary features are stressed as key factors for understanding the causes that control the trophic status of freshwater ponds. Fishes affect biodiversity, quality and water transparency. Restoration of eutrophic ponds must involve control of nutrient inputs and fish communities, with adequate policies for fish and plant management and conservation