Assessing carbon dynamics in natural and perturbed boreal aquatic systems

Most natural freshwater lakes are net greenhouse gases (GHG) emitters. Compared to 25 natural systems, human perturbations such as watershed wood harvesting and long term reservoir 26 impoundment lead to profound alterations of biogeochemical processes involved in the aquatic 27 cycle of carbon (C). We exploited these anthropogenic alterations to describe the C dynamics in 28 five lakes and two reservoirs from the boreal forest through the analysis of dissolved carbon 29 dioxide (CO2), methane (CH4), oxygen (O2), and organic carbon (DOC), as well as total nitrogen 30 (TN) and phosphorus (TP). Dissolved and particulate organic matter, forest soil/litter and 31 leachates, as well as dissolved inorganic carbon (DIC) were analyzed for elemental and stable 32 isotopic compositions (atomic C:N ratios, δ13Corg, δ13Cinorg and δ15Ntot). We found links between 33 the export of terrestrial OM to these systems and the dissolved CO2 and O2 concentrations in the 34 water column, as well as CO2 fluxes to the atmosphere. All systems were GHG emitters, with 35 greater emissions measured for systems with larger inputs of terrestrial OM. The differences in 36 CO2 concentrations and fluxes appear controlled by bacterial activity in the water column and the 37 sediment. Although we clearly observed differences in the aquatic C cycle between natural and 38 perturbed systems, more work on a larger number of water bodies, and encompassing all four 39 seasons should be undertaken to better understand the controls, rates, as well as spatial and 40 temporal variability of GHG emissions, and to make quantitatively meaningful comparisons of 41 GHG emissions (and other key variables) from natural and perturbed systems.