Hydrobiological characteristic of small lakes in northern Karelia during the ice-covered period

The life of lakes in winter is still insufficiently studied. The purpose of this article is a comparative analysis of the quantitative development of planktonic and benthic communities and the abiotic conditions of their existence during the ice-covered period in two small (<1 km2) northern lakes that differ greatly in morphometry and the degree of water humification. A significant difference was revealed in the seasonal dynamics of plankton communities in the studied lakes. In a deeper lake with water uncolored by humus, the quantitative development of phytoplankton at the end of the ice-covered period was almost as significant as during the open-water period. It is necessary to note the rapidity of changes in under-ice phytoplankton in early spring: in April 2020, the content of chlorophyll a in water changed more than six times during a week. There was no mass development of phytoplankton under the ice in a shallow highly humified lake. The zooplankton of the studied lakes was characterized by the opposite trend. In the deep-water lake, the winter zooplankton was noticeably inferior to the summer one in terms of the level of quantitative development; in the shallow lake, a high biomass (~1 g/m2) of zooplankton was noted throughout the entire study period. The macrozoobenthos of lakes in winter, on the whole, retained the same high level of quantitative development as in the open-water period. Seasonal differences were local and were caused by the redistribution of bottom animals due to changes in the oxygen and, possibly, temperature regime of bottom waters. In particular, due to hypoxic-anoxic conditions at the end of the ice-covered period, benthic animals migrated from the profundal zone of the deep-water lake. After the ice melted and the near-bottom waters were saturated with oxygen, the profundal community quickly recovered. Thus, the quantitative development of biological communities in winter can vary greatly even in closely located lakes, in particular, it depends on the degree of humification of the water body. Given the high dynamism of phytoplankton in early spring, the assessment of the role of the under-ice period in the life of lakes, apparently, requires long-term research, including detailed observations at the end of ice-covered period.