Interests and ecophysiological significance of the estimation of biomass and photosynthetic activity in three phytoplanktonic size fractions in an eutrophic lake
The seasonal distribution of phytoplanktonic biomass (estimated by biovolumes after cell counts and by chlorophyll a concentrations), primary production and primary production to biomass ratios (P/B"Counts" and P/B"Chlorophyll a") was studied by size fractionation in a eutrophic lake (lake Aydat, Massif Central). It appears that the 12-45 μm size fraction contributes about 50 % of the total phytoplanktonic biomass, regardless of what descriptor of the biomass is used and up to 65 % of the total photosynthetic activity. Clearly, the quantitative predominance of this fraction affects the water transparency (Zs) through the highly significant negative correlation obtained between Zs and the 12-45 μm chlorophyll a size fraction. The highly significant linear correlations obtained between the primary production of each fraction and surface to volume ratios (S/V) confirm the direct link between these two parameters. The mean values relative to the P/B of the < 12 μm fraction depend largely on the method used for the biomass estimates. First, the underestimation of the nanoplanktonic fraction by cell counts leads to an over-estimation of the < 12 μm P/B"Counts" fraction. Second, it is well known that chlorophyll a concentrations per unit volume are higher for nanoplankton than for larger cells. Thus, the use of the same biomass conversion factor for all the fractions, overestimates the nanoplankton biomass.