Abstract
Trends in thermal and oxygen conditions were identified in three stratified lakes that are part of the cooling system of two Power Plants. The total water uptake and discharge by the power plants decreased from approximately 60.0 m-3s-1 in 1995-1999 to approximately 50.0 m-3s-1 in 2005-2010, while, simultaneously, the degree of water heating decreased. Changes in the hydrological conditions influenced an increase in the mean and maximum water retention times in the lakes. Decelerated water flow rates and lower inertia decreased the thickness of the epilimnion and water temperature and oxygen content in Lake Licheńskie, the shallowest of the lakes studied. Oxygen conditions deteriorated in the deeper, isolated waters of Lake Ślesińskie. The more advantageous morphometric conditions in Lake Mikorzyńskie contributed to improved oxygen conditions in the near-bottom water layer and decreased levels of oxygen deficits in the spring-summer period.