We investigated radiatively driven under-ice convection in Lake Onego (Russia) during 3 consecutive late winters. In ice-covered lakes, where the temperature of water is below the temperature of maximum density, radiatively driven heating in the upper water column induces unstable density distributions leading to gravitational convection. In this work, we quantified the key parameters to characterise the radiatively driven under-ice convection: (1) the effective buoyancy flux, $B_{\ast }$ (driver), and its vertical distribution; (2) the convective mixed-layer thickness, $h_{CML}$ (depth scale); and (3) the convective velocity, $w_{\ast }$(kinematic scale). We compared analytical $w_{\ast }$ scaling estimates to in situ observations from high-resolution acoustic Doppler current profilers. The results show a robust correlation between $w_{\ast }$and the direct observations, except during the onset and decay of the solar radiation. Our results highlight the importance of accurately defining the upper limit of h_CML in highly turbid water and the need for spectrally resolving solar radiation measurements and their attenuation for accurate $B_{\ast }$ estimates. Uncertainties in the different parameters were also investigated. We finally examined the implications of under-ice convection for the growth rate of nonmotile phytoplankton and provide a simple heuristic model as a function of easily measurable parameters.

Oxygen conditions in ice-covered lakes depend on many factors, which, in turn, are influenced by a changing climate, so detection of the oxygen trend becomes difficult. Our research was based on data of long-term measurements of dissolved oxygen (2007–2020), water temperature, under-ice solar radiation, and snow-ice thickness (1995–2020) in Lake Vendyurskoe (Northwestern Russia). Changes of air temperature and precipitation in the study region during 1994–2020 and ice phenology of Lake Vendyurskoe for the same period based on field data and FLake model calculations were analyzed. The interannual variability of ice-on and ice-off dates covered wide time intervals (5 and 3 weeks, respectively), but no significant trends were revealed. In years with early ice-on, oxygen content decreased by more than 50% by the end of winter. In years with late ice-on and intermediate ice-off, the oxygen decrease was less than 40%. A significant negative trend was revealed for snow-ice cover thickness in spring. A climatic decrease of snow-ice cover thickness contributes to the rise of under-ice irradiance and earlier onset of under-ice convection. In years with early and long convection, an increase in oxygen content by 10–15% was observed at the end of the ice-covered period, presumably due to photosynthesis of phytoplankton.

This study investigated climate change impacts on watershed parameters as well as on water balance, water temperatures, and ice-cover regime for 2 large lakes in western Russia, Lakes Ladoga and Onego. Parts of the study occurred during a period of pronounced warming and limited ice cover on Lake Ladoga, which precluded winter surveys of the lake. This limitation notwithstanding, the research provides background climatic and hydrological conditions for the joint Russian–Swiss project “Lake Ladoga – Life under the ice.” For both Lakes Onego and Ladoga from 1955 to 2017, air temperature, precipitation, and evaporation data all showed increasing trends, and inflow, outflow, and water levels showed no discernible changes from baseline conditions. Thermal and ice conditions of Lake Onego showed greater sensitivity to climatic changes than corresponding conditions for Lake Ladoga because of the latter’s significantly smaller volume and lower heat content. The total ice-cover duration for Lake Onego decreased ∼20 days during the observation period. Detailed data analysis also revealed key characteristics of the Shuya River discharge into Petrozavodsk Bay, where a major part of the fieldwork occurred. River discharge into Lakes Ladoga and Onego increased in the winter. These processes collectively can change the hydrochemical conditions, water quality, and habitat.

The dependence of ice phenomena (the dates of ice freeze-up and break-up) on the air temperature and geographic factors (geographic latitude, height above sea level, and average depth and area of...

The statistical relationships between lake ice phenology (freeze and break-up dates, ice duration), air temperature and North Atlantic Oscillation (NAO) index are analysed for eight lakes in Karelia from 1950 to 2009. Linear trends over this time period are estimated. It is shown that in the last 20 years trends in the timing of ice phenomena are more evident than in the entire 60-year period. The statistical relationship between lake ice phenology and variability of regional air temperature is used in the empirical model to assess ice-related events on previously unstudied lakes in NW Russia.