Water-Level Fluctuations in Mediterranean Reservoirs: Setting a Dewatering Threshold as a Management Tool to Improve Water Quality

Abstract

Water-level fluctuations, often linked to seasonal climatic trends, are a natural phenomenon which occur in almost all aquatic ecosystems. In some climatic regions, as the Mediterranean one, they are particularly wide due to the occurrence of two well separated periods: the rainy winter and the almost completely dry summer. Precipitation is concentrated in the first period, whereas in the second strong evaporation losses take place. According to these climatic features, and to ensure a continuous supply of water throughout the year, man-made lakes store water during winter and are subjected to dewatering during summer to compensate the lack of precipitation. These ecosystems are thus characterised by rather wide water level fluctuations which were observed to transform them from potentially warm monomictic lakes into polymictic or atelomictic ones. These changes deeply affect the biological structure and the functions of the water bodies impairing the response of some ecosystem properties, as resilience and resistance, since the impacts are immense enough to move the systems out of their homeostatic plateau of, respectively, deep or shallow lakes. In order to understand to what extent a reservoir can be “emptied” without changing its ecosystemic identity (deep or shallow lake sensu Padisák & Reynolds, 2003) and to set a “dewatering threshold”, the results from two different hydrological years, one with a dewatering so intense as to disrupt thermal stratification in midsummer, and the other one with water enough to allow the maintenance of the reservoir’s thermal structure throughout the summer, are compared. Former investigations have shown that the persistence of thermal stratification has a positive value in Sicilian reservoirs: a notable decrease in total phytoplankton biomass and in the relative occurrence of cyanoprokaryotes was observed in the high-level year with a stable thermal stratification. Although the solving of the external load problems causing eutrophication phenomena remain the main task to improve the water quality of this Mediterranean island, a management procedure, based on the maintaining of the ecosystem within its homeostatic plateau through the setting of a dewatering threshold, is suggested.