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Abstract

The aim of this study is to conclude the experiences of the maintenance practice of an artificial thermal lake. The first years (2015-2017) of the development of the lake were analysed and evaluated along the following questions: a) What kind of design processes and maintenance interventions are related to the process of fitting the lake into the landscape? b) How have the ecological conditions of the lake evolved in the past period (zonation, succession, plantation and colonization)? c) Which general experiences could be gained from the operation of this artificial thermal lake this far? The authors of the present study were already engaged in the planning process, participating in the preparation of four different design documentations. The planting design and the maintenance instructions were based on a physico-chemical monitoring, phytoplankton, zooplankton and macroinvertebrate sampling, and machrophyte assessment. The significant processes during the three years of the lake are presented by functional groups of biota, separately assessing the characteristics of the changes of macrophytes. In 2017 an individual macroinvertebrate assessment was done, moreover a fish die-off occurred in August 2017, which are mentioned separately as well. The data in total suggested that the water of the lake is highly hypertrophic, further machrophyte introduction can prevent the plant nutriments to be absorbed by algae. Partly the algal growth but also the unlucky coincidence of other factors (e.g. high water temperature, cold weather front, maintenance problems) led to the die-off of the spontaneously overpopulated fish stock in 2017.

Thermal and oxygen conditions in lakes under restoration following the removal of herbivorous and seston-filtering fish

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.

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