A field study on current structure and circulation characteristics in Lake Vendyurskoe, a small, shallow, icecovered lake in Karelia, Russia, is presented. The current velocity magnitudes were generally found to be small. The most pronounced currents had an oscillating character, with velocity amplitudes on the order of millimeters per second. The oscillation period, obtained from spectral density calculations, corresponded to that of a barotropic uninodal seiche. The seichelike nature of the current oscillations was supported by the results from analysis of icelevel fluctuations, giving identical periods and a phase shift of one-fourth the period between the two types of oscillations. Mean currents measured during the winter were on the order of millimeters per second. Because Lake Vendyurskoe does not have any significant river inflow or outflow during winter, the most probable cause of these currents is horizontal temperature (pressure) gradients. Scaling analysis indicated that these currents are geostrophic. This was supported by theoretical estimates, based on observed horizontal temperature gradients, being of the same order as the observed currents. The mean current velocities increased considerably after spring convection from

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.

A field study was carried out in three small shallow ice-covered lakes to study heat and mass fluxes and their spatial and temporal variability. During the main part of the winter, the heat flux at the ice-water interface, being of the order 0.5-1 W m−2, was dominated by conduction from water to ice and did not show any significant variations in time or space. The heat flux from sediments to water was the main source for the lake water heating during early and mid-winter, being depth-dependent and 1-4.5 W m−2 in early winter, and 0.5-3 W m−2 in late winter. A heat transport from shallow regions to deep parts was shown to occur during the winter, being of the same order as the vertical fluxes, and should thus be accounted for in any attempt to predict the temperature evolution in an ice covered lake. The salt flux from sediments was found to be of the order 1-10 × 10−10 kg m−2 s-l. A comparison of this flux with salt content changes indicates that the former is of the same order as the horizontal salt flux which is directed from shallow regions to the deeper parts of a lake during winter.