Abstract
The nutrient conditions in the oligotrophic lake Llyn Ogwen were compared with the populations of algae over a period of 27 months. Oedogonium suecicum (Wittr.) Hirn and a species of Mougeotia (sterile) were correlated with the level of phosphate, but a succession of filamentous desmids could not be correlated with any measured factor. The factors controlling populations of Dinobryon sertularia Ehr. and Tabellaria flocculosa (Roth) Ktz. in Llyn Ogwen were discussed and compared with previous records.