Effects of climate, fire and vegetation development on Holocene changes in total organic carbon concentration in three boreal forest lakes in northern Sweden

Near infrared spectroscopy (NIRS), diatoms, pollen, charcoal, loss-on-ignition (LOI), and nutrient elements in lake sediments were used to assess important factors controlling Holocene changes in the total organic carbon (TOC) concentration, pCO2, color and pH of lake water in three boreal forest lakes in northern Sweden. The results suggest that mire formation, fire frequency and humidity are the most important forcing factors on millennial timescales. Mires produce humic acids that become available to the lakes, whereas fires may reduce the pool of carbon in the catchments, and humidity controls the transportation of allochthonous carbon into the lakes. Vegetation development and temperature as sole factors are of minor importance for the TOC concentrations in these lakes on a millennial timescale. Two of the sites indicate that liming and possibly fish introduction and rotenone treatment in recent time has led to increased TOC, color and pH in the lake water, and changed the diatom community composition to an assemblage that has never been present before. Given the predicted climate change scenario that suggests a more humid climate, expanding mires and less frequent fires, our paleolimnological data suggest that TOC concentrations can be expected to increase in boreal forest lakes in the future. Since super-saturation and emission of CO2 from lakes is correlated to the TOC concentration of lake water, higher TOC concentrations may lead to increased emission of CO2 from lakes to the atmosphere.