Seasonal variations of phytoplankton photosynthate partitioning in two lakes of different trophic level

The aim of this study was to compare vertical and seasonal variationsof inorganic carbon allocation into macromolecules by the phytoplanktonpopulation in a eutrophic lake (Lake Aydat) and an oligo-mesotrophiclake (Lake Pavin). Biochemical fractionation was conducted byconsecu tive differential extractions in order to separate proteins,polysaccharides, lipids, and low molecular weight compounds(LMW). The ratio of light absorption at480 and 665 nm by acetoneextracts of phytoplankton pigments was used as an indicatorof the nutritional statusof natural phytoplankton populations.Our results show that in Lake Aydat, the main photosyntheticend productswere poly saccharides, whereas in Lake Pavin, radioactivitywas predominantly incorporated into the protein fraction. Moreover,the seasonal cycles of mixing and stratification in these twolakes affected the pattern of ¹⁴C incorporation into LMW andmacromolecules. An increase in the relative synthesis of proteinsoccurred during stratification periods. It was linked to anincrease in temperature and nutrient limitation further complicatedby the shift in species composition of the populations. Differences recorded both between the two lakes of different trophicstatus and between seasons confirm that the proportion of carbonincorporated into proteins might be a useful indicator of thephysio logical status of phvtoplankton communities.