Seasonal variation in the filament morphology of the freshwater diatom Melosiragranulata(Ehrenb.) Ralfs

SUMMARY 1, The seasonal changes in filament dimensions and the occurrence of separation valves in a population of Melosira granulata(Ehrenb.) Ralfs were investigated in a eutrophic lake in southern England. Experiments were carried out in culture to study the effects of various environmental factors on the diatom's morphology. 2. Major blooms of M. granulataoccurred from September to October, coinciding with an increase in mean filament length brought about by the high growth rate of the diatom. This increase was controlled by a reduction in the frequency of separation valve production. A similar increase in filament length occurred in rapidly growing cultures. 3. The population was lost from the epilimnion when thermal stratification occurred. Immediately prior to this the population consisted of short filaments. 4. The mean filament length was greatest during occasional summer blooms and was again controlled by a reduction in the frequency of separation valve production. This appeared to be a response to the reduced nutrient concentration in the lake. This is supported by similar morphological changes in low nutrient cultures. 5. Increases in the filament length of the diatom were accompanied by increases in the diatom's sinking velocity. The length increase thus ensured its rapid return to the sediment, after summer growth periods, where the cells could be replenished with nutrients.