Greenhouse gas metabolism in Nordic boreal lakes

Summary

Boreal lakes are important net sources of greenhouse gases (GHGs). In this study we analyzed concentrations of CO2, CH4, N2O as well as O2, N2 and argon (Ar) from the epilimnion of 75 boreal lakes covering gradients in total organic carbon (TOC), phosphorus (P) and nitrogen (N) deposition. The Ar-corrected gas saturation deficit was used as a proxy of net metabolic changes from spring overturn to mid-summer sampling (all lakes were dimictic). Emission fluxes were calculated for CO2, CH4 and N2O based on partial pressure, water temperature and wind speed. Gas concentrations, actual and Ar-corrected, were related to lake specific properties. TOC was the main predictor of CO2 concentrations and fluxes, followed by total P, while total P and chlorophyll a governed CH4 concentrations and fluxes. Nitrogen (NO3 - or total N) were key predictors of N2O concentrations and fluxes, followed by total P. Altitude, area and depth were not strong predictors of CO2, CH4 and N2O concentrations and fluxes, likely because only lakes with an area of [1 km2 were included. CO2 molar concentrations were negatively correlated with O2 concentrations, while the slope of CO2 concentration to Ar corrected O2 deficit was 1.039. Together with the poor correlation between area-specific primary production and CO2 as well as O2, this suggests that these gases are mostly affected by catabolic processes and probably photo-oxidation in these nutrient-poor, boreal lakes investigated in this study. Increasing inputs of TOC (i.e. lake ‘‘browning’’) is likely to promote the net heterotrophy and hence emissions of all GHGs, while elevated N deposition in particular may cause elevated emissions of N2O.