Changes in physico-chemical characteristics and the succession of phytoplankton in the lake Velenjsko jezero following its restoration

The species composition of phytoplankton in the artificial lake Velenjsko jezero has been monitored since 1994 while physico-chemical characteristics of the lake water since 1998. Before the year of lake remediation, 1994, the pH of lake water was around 12. In 1994, only filamentous cyanobacteria Oscillatoria ssp. were present in high abundance, with the rare appearance of Synedra sp. and Ceratium sp.. In 1995, the pH in the upper water layers decreased to 9, as a consequence of the construction of a fly ash system with a closed loop water cycle in October 1994. The number of algae taxons increased to 7 (Coelosphaeria sp., Gomphosphaeria sp., Scenedesmus sp., Pediastrum sp., Asterionella sp., Synedra sp. and Ceratium sp.). In 1996, when the pH fell to 8, it increased to 13. The lake provided good conditions for algal development since it was rich in nutrients. Since 1996 the level of nutrients in the upper layers of the water column has remained more or less the same, but in the deeper layers the reduced form of nitrogen (NH4+) has increased and the oxygen curve has become clinograd. Velenjsko jezero can be classified according to OECD, as hypereutrophic on the basis of the level of total phosphorus (120 µg L–1) and total nitrogen (1500 µg L–1), the average transparency of 5.38 m corresponds to mesoeutrophic status, and the average concentration of chlorophyll a at 1.03 µg L–1 to oligotrophic status. Despite the high availability of nutrients the primary production was not as high as in a similar natural lake ecosystem, which could be ascribed to the high concentration of ions Ca2+, K+ , Mg2+, Na+ , Cl– and particularly, SO4 2–. The predominant algae in the lake in 2007 were cyanobacteria Pseudanabaena cf. catenata, Planktothrix rubescens, from which the first bloom occured in June and the second from November to January, and dynophyta Ceratium hirundinella and Peridinium cinctum.