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 presenceof 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.