The life cycle of the copepod Thermocyclops oithonoides (Sars, 1863) in lake Onega

A study of the annual cycles of plankton community and its main elements is of particular relevance in the conditions of climate change and anthropogenic load. The copepod Thermocyclops oithonoides (Sars, 1863) is the main component of the pelagic zooplankton of Lake Onega and a valuable component of planktivorous fish diet. The life cycle of the crustacean populations from the central oligotrophic region of the lake and the large Petrozavodsk Bay was analyzed on the basis of long-term observations. T. oithonoides is found in plankton all year round. In winter, only stage IV-V copepodites are marked, which overwinter in the bottom layer. During the subglacial period, the abandance of the crustaceans reaches 25,3±4,8 th.ind./m3, biomass - 0,286±0,055 mg/m3. It was shown that population dynamics of T. oithonoides in the central part of the reservoir is dicyclic. In the bay, because of the favorable temperature regime, the crustacean can have di-and three cycles. The average long-term population maximums in the center of the lake reach 1,31 th.ind./m3 (abundance) and 0,009 g/m3 (biomass). In the Petrozavodsk Bay the values of population abundance and biomass of T. oithonoides are similar (1,36 ind./m3 and 0,013 g/m3, respectively). In June, adverse hydrochemical conditions in the bay inhibit the development of the crustaceans and cause a delay in its quantitative development. Comparison with the data of the last century showed that, despite the fluctuations in anthropogenic load and climate changes observed in recent decades, no noticeable changes in the nature of the annual cycles of Thermocyclops oithonoides have occurred.