RESUMO
Large and temperate Lake Peipsi is the fourth largest lake in Europe, where the massive cyanobacterial blooms are composed mostly of Microcystis spp., which have been common for several decades now. The seasonal dynamics of potentially toxic Microcystis were studied using microscopy and quantitative polymerase chain reaction (qPCR) by assessing the microcystin-encoding microcystin synthetase gene E (mcyE) abundances. Water samples were analyzed over the lake areas, varying in depth, trophic level, and cyanobacterial composition during the growing period of 2021. The Microcystis mcyE genes were detected through the growing period (May-October), forming peak abundances in September with decreasing temperatures (8.9-11.1 °C). Total phosphorus (TP) and nitrate (NO3-) were the most relevant environmental variables influencing the Microcystis biomass as well as mcyE abundances. Comparison with previous years (2011, 2012) indicated that the abundance and seasonal dynamics of toxigenic Microcystis can be highly variable between the years and lake areas, varying also in dominant Microcystis species. Contrary to expectations, based on mcyE abundances, the increased risk of toxin-producing Microcystis can occur in Peipsi through the growing period, independently of the water temperature and biomasses of Microcystis.