Resting stages and ecology of the planktonic diatom Aulacoseira skvortzowii in Lake Baikal

Populations of the planktonic diatom Aulacoseira skvortzowii in Lake Baikal developed below 4 degrees C, with mortality increasing rapidly at temperatures above 6.5 degrees C. 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 mu g L-1 P-PO4. 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.