Relative contribution of bacteria, algae and ciliates to the microplankton biomass in an eutrophic lake

Abstract

AbstractBiomass in size‐fractionated plankton (0.2–1.2 μm; 1.2–12/μm; 12–45 μm and 45–160 μm), together with that of ciliated protozoans (size fraction 1–160 μm), were assessed during stratification, in an eutrophic lake (Lake Aydat, Massif Central, France). Samples (51) were gathered from depths of 2, 7 and 14 m during 17 weekly samplings in the centre of the lake. Three biomass estimations were performed: cell‐counts, chlorophyll a and ATP assays. Ciliates were counted live.Five species groups dominated the ciliate community: Loxodes striatus, Frontonia spp., Spirostomum ambiguum, Strombidium spp. and Dexiotrica plagia. Frontonia always had the highest cell concentration (54–64% of total cell counts), while the bulk of biomass was divided between Loxodes (42–70% of total) and Frontonia (29–53% of total).In terms of carbon, both the bacteria and ciliate contributions to the total microplanktonic biomass (size fraction 1–160 μm, i.e. bacteria + phytoplankton + ciliates) depends on the choice of the phytoplankton biomass estimation technique used. Thus, the contributions increase respectively from 16% and 19%, when phytoplankton biomass is estimated from chlorophyll α concentrations, to 24% and 27%, when phytoplankton biomass is based on biovolume.Relationships between bacteria, phytoplankton and ciliated protozoans reveal that ATP data are greatly overestimated by bacteria and ciliate interferences, especially in the metalimnion and hypolimnion. In the epilimnion, this interference is less. Interpretations of phytoplankton dynamics and metabolic activity from ATP assays should be made with extreme care, especially in eutrophic environments.