Vertical mixing, size change and resting stage formation of the planktonic diatom Aulacoseira baicalensis

Aulacoseira baicalensis (K. Meyer) Simonsen is a freshwater planktonic diatom that undergoes large seasonal changes in cell morphology related to changes in vertical mixing. Short cells (10–20 µm) with thin walls were formed under the ice of Lake Baikal but cell lengths increased up to 150 µm by the time mixing depth reached over 100 m in June. These long cells became resting stages that were packed with reserve products and had siliceous walls up to 4 µm thick. Increase in mixing depth gave access to sufficient silica for completion of resting stages in most years but not in high biomass years, which has long-term implications for the population. Wall thickening reduced the risk of dissolution during dormancy but it also reduced cell volume. Therefore, by increasing length, cells maintained storage space for reserves. Seasonal changes in valve length showed that individual valves did not last more than 6 months, equivalent to 5 to 10 divisions. Separation valves were important in determining the number o...