P. rubescens is likely to bloom in Lake Bourget following a conjunction of factors and processes. Firstly, the presence of an autumn/winter innoculum of the cyanobacterium the year before is significant. Secondly, the phosphorus resource remains a key issue and severe limitation in winter/spring of the year can prevent the bloom whereas presence or input (i.e., P already present in surface waters of the lake or coming from tributaries or from the sediment) during the winter/spring of the year can be crucial. Moreover, a relatively high level of winter irradiance and/or water transparency may clearly favor the initial development of P. rubescens merely at depth (i.e., between 15 and 25 m).