Limnological aspects of sicilian reservoirs: A comparative, ecosystemic approach

Abstract

This paper is a report on the state-of-the-art of reservoir limnology in Sicily, the largest island of the Mediterranean Sea. The territory is characterised by a semi-arid climate with a dry and a wet semester occurring in the year. Evaporative losses are strong during the summer and, on an annual basis, they often exceed precipitation. Thus, reservoirs constitute an important water resource for the island to supply drinking water and to provide for agricultural needs in the periods of drought. However, despite their importance, they receive poor attention from the scientific and political communities and in the absence of any environmental policy or suitable territorial planning, their water quality is undergoing a rapid decline, principally due to the inadequate treatment of urban waste waters and to the excessive and irrational spreading of fertilizer. The convinction that reservoirs may differ significantly from lakes and the necessity to better understand what rules drive the functioning of these environments, were the engine to carry out several limnological investigations on these artificial environments with special reference to their pelagic compartment. The principal characteristics of these water bodies lie in their peculiar hydrological cycle. The absence of precipitations during summer, when the reservoirs are intensively used, and their re-filling in winter without any outflow contribute to water-level fluctuations which interfere with their thermal structure, underwater light climate and nutrient dynamics'. The structure of phytoplankton assemblages is, in the absence of a severe nutrient limitation, modulated by hydrological forcing. In addition to these "bottom-up" effects some "top-down" ones, also mediated by the water-level fluctuations. These involved the strength of predation pressure exherted by fish on zooplankton assemblage. Ultimately, it has been demonstrated that these water movements influence the structure of these ecosystems profoundly and their effects have to be carefully considered when management procedures have to be developed.