LIFE CYCLE OF A STEPHANODISCUS SP. (BACILLARIOPHYTA) 1

Studies of the life cycle of a centric diatom, tentatively identified as Stephanodiscus neoastraea Håkansson & Hickel, showed that sexual reproduction occurred every year in a freshwater lake (Lough Neagh, Northern Ireland). Male and female gametes were produced in cells below 55% of the maximum diameter during a 3–4‐week period in late summer, following the return of nitrate concentrations above 10 μM NO 3‐ N. The frequency of sexual reproduction was linked to the cycle of diameter size reduction and regeneration. The times of largest decreases in cell diameter were during nutrient stress in summer and low light conditions in late autumn, rather than during the main spring growth period. So, environmental conditions (combined with the limited life‐spans of individual cells) affected the rate of diameter reduction and, therefore, the length of the life cycle (3–4 years).