The annual cycle of phytoplankton biomass was followed in a eutrophic lake (Lake Aydat, Massif Central, France), using classic descriptors (biovolumes and chlorophyll a) as well as adenosine-5'-phosphate (ATP) levels. ATP is the metabolite indicating living biomass. ATP/Cell count and ATP/Chlorophyll a ratios were elevated due to the presence of heterotrophic organisms, such as several species of ciliates. The first ratio is greater than the second. The difference can be explained either by an underestimation of the nanoplankton fraction through the cell count method, or by a lack of sedimentation in the settling chambers used for counting cells.