Spatial and temporal variations in autotrophic and heterotrophic compartments of the microbial loop in southern Quebec lakes
The biomass of autotrophic and heterotrophic microbial compartments were measured in nine lakes in two regions of southern Quebec. Six lakes were located in the Laurentides while three lakes were situated in the Eastern Townships. They varied in trophic status, watershed geology, and water chemistry. Each lake was sampled at three depths, corresponding to the epi-, meta-, and hypolimnion strata, and at five dates over the summer season (1991-1992). The mean biomass of autotrophic picoplankton (PPA: 16-80 μg·C·L-1), heterotrophic picoplankton (PPH: 97-647 μg·C·L-1), autotrophic nanoplankton (NPA: 7-37 μg·C·L-1), and heterotrophic nanoplankton (NPH: 9-29 μg·C·L-1) found in Quebec lakes over a PT range of trophy varying from 7 to 73 μg·L-1 were of the same order as those reported in other freshwater environments in Canada. Most of the variation in the biomass of the four microbial compartments was related to among-lake variation. Some variation was also due to within-lake spatial variation among limnetic strata or temporal variation over the summer season. However, within-lake patterns of variation patterns were different among lakes. Among-lake variations in the biomass of the four microbial compartments were examined in relation to changes in total phosphorus concentrations (PT) and macrozooplankton densities between lakes. Both lake trophy, expressed by PT, and macrozooplankton abundance influence the biomass of picoplankton compartments. The biomass of PPA and PPH decline in the most eutrophic lakes of the Eastern Townships where PT > 20 μg·L-1. PPA and PPH biomass were also reduced in lakes with the highest density of macrozooplankton or with low micro-/macrozooplankton ratio. Our study did not clearly detected the effects of lake trophy or macrozooplankton on nanoplanktonic compartments.