Studying and forecasting the consequences of the climate change impact on aquatic ecosystems are among the important tasks of hydroecology and hydrobiology. Over the past 30 years, according to data of meteorological stations on the catchment area of Lake Onego–the largest reservoir of Northwestern Russia—the average annual air temperature is steadily increasing. In the winter of 2019–2020, an abnormally high air temperature was recorded, which exceeded the climatic norm by 5–9 °С. According to satellite data, in winter 2020, for the first time in the 65-year history of observations, there was no ice cover on most of the open part of Onego. To assess the consequences of an abnormally warm winter for the ecosystem of Onego, its state in the spring–summer period was studied. In June 2020, the water temperature, the concentration of chlorophyll “a,” and photosynthesis were measured, and the structural parameters of phytoplankton in Onego were studied using standard methods. Based on the data from the temperature sensors of the buoy station, it was found that the spring thermal bar passed 10–15 days earlier than the average long-term periods, the temperature stratification of water was established 15–20 days earlier. The temperature of the surface water layer in Onego in June 2020 was 5–8°C higher than the average long-term values for the period of spring mixing and corresponded to the period of summer heating of water. In the open part of the lake, a shift in the phenological phases of phytoplankton development was observed, in particular, its summer composition, the seasonal maximum of chlorophyll “a,” and primary production were formed a month earlier. In the Kondopoga Bay of Onego, which was exposed to the highest anthropogenic phosphorus load, the level of phytoplankton development first reached the a-eutrophic state.