Effect of snow depth on under-ice irradiance and growth of Aulacoseira baicalensis in Lake Baikal

Lake Baikal freezes for 4-5 months each year; 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 mu mol 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 mu m) diameter cells trapped in high light intensities in sub-ice layers switched to auxosporulation and size regeneration.