Temperature and Salt Content Regimes in Three Shallow Ice-Covered lakes

Abstract

A field study on the temperature, salt content, and density regime in three shallow ice-covered Karelian lakes is presented. The measurements show that the heat content increases during the whole ice-covered period. At ice formation a weak stable stratification existed in the lakes, with average temperatures about 1°C. Thereafter, the stability of the stratification gradually increased, mainly due to pronounced temperature increases in the bottom layers. In mid-winter the bottom layer in the deep parts of the lakes obtained temperatures above 4°C. The density stratification in these layers was stable, however, due to higher salt contents (increasing continuously during the winter) in the vicinity of the bottom. The horizontal variations in temperature and salt content were very small, and both parameters can be considered to be horizontally homogeneous. Under-ice convection was developed in two of the three investigated lakes during the second half of April, when heating due to penetrating solar radiation became apparent. Although no under-ice convection in the conventional sense occurred in the third lake (Uros), interior convection developed when the temperature exceeded 4°C (the temperature of maximum density) there. The absence of under-ice convection in Lake Uros is most likely due to the higher vertical temperature gradient in the lake before spring heating and smaller extinction coefficient than in the other two lakes.

Field study on currents in a shallow, ice‐covered lake

“…All other instruments were developed at the Northern Water Problem Institute. For a thorough description of these devices, see Glinsky (1998) and Malm et al (1996). perature differences may affect currents during the winter, a brief summary of these results is given below.…”

“…The data were reported by Malm et al (1996). Stations, dates and duration of measurement, and depth of the current meter sensor for these long-term registrations are given in Table-2.…”

“…Measurements included (1) thermistor chain registrations; (2) temperature and conductivity profiling in full verticals (49 stations), directly under the ice cover (10 stations), and at the sediment-water interface (~ 10 stations); (3) temperature gradients in the upper layer of bottom sediments (~ 10 stations); (4) current velocity registrations (~ 10 stations); and (5) sediment characteristics (type, porosity, and organic content). The measurement data were reported by Malm et al (1996), and the results of temperature structure and dynamics and boundary heat fluxes were reported in two later articles (Malm et al 1997a,b). The current velocity measurements during each survey included (1) mean current velocities at stations along cross section 4 and (2) long-term observations of current velocities and ice-cover movements (vertical and horizontal) at a few stations.…”

“…The horizontal ice-displacement meter consists of a wire on a moored float (fixed relative to the bottom), which is sensed by a unit of four electrodes on the ice surface (calibrated in the range of -15 to + 15 mm). Both instruments are described more thoroughly by Malm et al (1996). When making current measurements at various depths, to allow for varying thicknesses of the ice, we referred the depth to the surface of open water (in a hole in the ice).…”

137Cs in Lake Tapeliai, Lithuania

“…Considering that a completely mixed water column is isothermal and unstable [39], cold weather and further winds may lead to a significant overcooling of the water column below the temperature of water maximum density (~4 °C). Thus, in Nordic lakes, average temperatures of the water column before the formation of the ice cover drop down to ~0.5 °C [41,42]. In this case, a 4 °C isotherm must be deepened below the sediment surface due to elevated thermal diffusivity of the sediment surface layer, which is affected by the buoyancy of interstitial liquids [23,43].…”