Surficial sediments from 76 lakes from two western Quebec regions (Abitibi and Haute Mauricie) were sampled to identify the relationships between diatoms and environmental variables. Because the two regions contained radically different diatom communities, we then investigated which factors may be responsible for the large community discrepancies in the two nearby geographical areas. Standard lake chemistry variables showed little differences between the regions, although epilimnetic light regimes were slightly lower in Abitibi. Nevertheless, lakes of the two regions with similar light regimes and chemistry still showed a clear separation in their diatoms, implying that other important factors are influencing assemblages. We found that the calculated concentration of CO2 in the open water can explain some of the discrepancy in diatom assemblages. A pCCA constrained to the concentration of CO2 with alkalinity and pH as covariables explained 12.5% of species variance and was significant. Given the lack of a relationship between DOC and CO2, and because the lakes are heavily supersaturated with CO2 in the calibration set, lake-to-lake variations in CO2 concentrations are likely due to groundwater inputs; the possibility that this environmental variable may be influencing diatom communities might allow, in some cases, the reconstruction of historical changes in groundwater inputs to lakes. Finally, new calibration models were built in Quebec by using weighted averaging partial least square (WA-PLS) techniques in order to infer pH, CO2, TP, TN, and, DOC from diatom assemblages preserved in the surface sediments.
