Controls of organic and inorganic carbon in randomly selected Boreal lakes in varied catchments

Organic and inorganic carbon concentrations in lakes and the links to catchment and water quality were studied in variable landscapes using the Finnish Lake Survey data base including 874 randomly selected lakes sampled during autumn overturn. The median total organic carbon (TOC) in these boreal lakes was 7.8 mg l−1, the median total inorganic carbon (TIC) 1.6 mg l−1 and the median partial pressure of CO2 (pCO2) 900 μatm. When the data was divided into subgroups according to land use in the catchment, the proportion of TIC of the total carbon (TC) in lakes was highest (31%) in agricultural areas and lowest (10%) in peatland areas. Elevated TIC concentrations were associated with agricultural land in the catchment, whereas elevated TOC concentrations were observed in lakes with high peatland proportion in the catchment. Two contrasting important sources of CO2 in lakes were identified on the basis of statistical analysis of the data; weathering processes in the catchments and decomposition of organic matter. CO2 was also strongly associated with total nutrients TN and TP, implying the importance of quality of organic matter and availability of nutrients for the decomposition processes.