Mediterranean Climate and Eutrophication of Reservoirs: Limnological Skills to Improve Management

Sicily is the largest Mediterranean island and one of the most densely populated areas in the region. To counteract the periodic oscillation in water availability and fulfill drinking and agriculture needs, about 30 dam-reservoirs, impounding in total 750 106 m3, were built in the last 60 years. However, the inherent features of the Mediterranean climate and the total lack of awareness from local authorities have contributed to a progressive increase in the trophic state of these human-made lakes. In particular, the water is stored during the winter rainy season and then it is intensively used in the dry summer. These operational procedures, in particular the time separation between filling and emptying phases contributes to increase the internal loadings. Under this hydrological regime, reservoirs act as a sink for nutrients. Moreover, the wide water-level fluctuations which characterize these environments further contribute to enhance eutrophication processes by promoting the instability of the thermal stratification and by establishing a state of atelomixis during the summer. Consequentially, most Sicilian reservoirs are affected by intense cyanobacterial blooms, which impair their water quality and cause concern due to their toxicity. Since a reduction of external loadings, which involves economic and political decisions, is far from being achieved, some procedures based on the management of the hydrological patterns of these reservoirs have been proposed. These procedures are aimed at avoiding excessive dewatering and at maintaining summer stratification. A reduction in phytoplankton biomass and a structural change in phytoplankton composition with a reduction of toxic cyanobacterial blooms have been observed as a result of the implementation of the above mentioned procedures