Increase of density due to mixing (cabbeling instability) in a spring thermal bar in Lake Baikal results in the sinking of cold surface water (T = 3.2−3.4°C, w = 0.3 cm s−1) below the profile of Tmd. This density increase generates forced convection and subsequent movement of cold water along the underwater slope to maximum depths. Cooling of deep and near‐bottom waters caused by forced convection is compensated during other seasons of the year by eddy heat flux directed toward the bottom. On a long‐term basis, vertical temperature profiles in deep waters remain constant.

Lake Baikal freezes for 4-5 months eachyear; yet the planktonic diatoms that grow under the ice include some of the largest species found in freshwater. An important factor influencing their growth is the depth of snow. In this study, a population of Aulacoseira baicalensis developed where there was little or no snow on the ice but declined where there was 10 cm of snow, because 99% of the available light was attenuated. Culture studies of light response showed that A. baicalensis was adapted to relatively low light intensities (\40 lmol m -2 s -1 ) that were close to the average that a cell experiences in L. Baikal when mixed vertically by convection to depths that can exceed 100 m. On sunny days, cell division could be inhibited down to [10 m depth but narrow (\15 lm) diameter cells trapped in high light intensities in sub-ice layers switched to auxosporulation and size regeneration.

Populations of the planktonic diatom Aulacoseira skvortzowii in Lake Baikal developed below 4uC, with mortality increasing rapidly at temperatures above 6.5uC. Resting spores were produced before the temperature rise associated with summer stratification. The main cue for sporulation was a decline in phosphate concentration below 15-20 mg L 21 P-PO 4 . If phosphate declined after the onset of stratification, sporulation was poor. In culture, all cells sporulated when phosphate limited but only 15% did so when nitrate limited. Also, spore formation was diameter dependent, with most narrow cells switching to size regeneration. This affected population dynamics, with high biomasses developing in the south and middle basins but only rarely in the north basin, because phosphate did not always fall below the induction threshold necessary for sporulation and size regeneration, leading to poor recruitment. In culture, germination occurred when spores were placed in new media, with stored reserves sufficient to complete two to three divisions, even in the dark. This helped populations re-establish when resuspended by wave action from coastal sediments where they lay dormant during summer.

No abstract