Man-made lakes in Mediterranean semi-arid climate: the strange case of Dr Deep Lake and Mr Shallow Lake

The lack of any protection against eutrophication is progressively compromising the water quality of Sicilian reservoirs. These water bodies provide the population with an adequate supply of drinking water and support most of the irrigation requirements of local agriculture. Moreover, they respond to the Mediterranean climate, with sequential, seasonally predictable events of flooding and drying and whose intensity varies markedly between years. As a consequence of summer drought and the policy to meet water demand throughout the season, the reservoirs experience massive dewatering, resulting in a reduction in the spring storage volume of 90%. Thus, they start the hydrological season as water bodies deep enough to form stable stratification but a thermocline has generally disappeared by the middle of summer and, as a consequence of the intense drawdown, they are transformed into shallow water bodies. This dual behaviour, driven by a progressive and conspicuous depth decrease, interferes with nutrient and phytoplankton dynamics, enhancing eutrophication phenomena and contributing to the selection of planktic cyanobacteria that are potentially dangerous to human health.