Abstract
The thermal and oxygen conditions were determined in lakes under restoration after the removal of herbivorous and seston-filtering fish. Studies of the thermal and oxygen conditions were conducted from March to November from 2000 to 2010, except in 2006, in lakes Dgał Wielki, Dgał Mały, and Warniak. The beginning of thermal stratification was usually observed in dimictic lakes in May, although this type of layering was noted several times by the end of April. Lake Dgał Mały has characteristics that are typical of so-called cold thermal lakes, while the characteristics of Dgał Wielki identify it as a thermally moderate lake. Oxygen depletion was observed in summer in the hypolimnion and metalimnion (clinograde curve) and H2S occurred in the deepest water layers, which was noted in the 1950s; this indicates that this occurred prior to, during, and following the experimental introduction of fish. Oxygen maximums have been noted in the metalimnion sporadically (positive heterograde). The water masses in the polymictic Lake Warniak warmed most quickly in spring and summer and cooled in fall. Homothermia is usually noted in the growth season, with good oxygenation from the surface to the bottom with frequent occurrences of supersaturation, which could signal intense production processes. The studied lakes differed significantly in terms of water mass warming, oxygen content, and degree of staticity.