Carbon pools and fluxes in a chain of five boreal lakes: A dry and wet year comparison

Lakes play an important role in catchment carbon (C) balances. However, the role of lakes in landscape C cycling in changing climate is highly uncertain. We studied C pools in a chain of five boreal lakes and compared the C fluxes during a wet and a dry year. The included pools were dissolved inorganic carbon (DIC) and total organic carbon (TOC) in lake water, as well as C pools in littoral macrophytes and lake sediments. The estimated C fluxes consisted of hydrological input and output of DIC and TOC, emission of carbon dioxide (CO2) to the atmosphere, DIC incorporated as organic carbon in primary production of phytoplankton and littoral macrophytes, and sedimentation of C. The riverine input of C into the lakes increased remarkably (40-210%) in the year with high precipitation. Simultaneously, there was a clear increase in the flux of CO2 to the atmosphere from the three small uppermost lakes, whereas in two large lowland lakes the CO2 fluxes were higher during the warm dry year. On the landscape scale, the role of small lakes (area <1 km(2)) was emphasized in net C accumulation, whereas the large lowland lakes released more CO2 to the atmosphere. In parallel, the concentration of TOC in the water column decreased downstream the lake chain whereas DIC increased. The largest pool of C (>98%) in all the lakes was in the sediment. Sediment C store, calculated relative to the lake area, was positively correlated with lake water TOC (r(2) = 0.93, P < 0.05) and Fe (r(2) = 0.96, P < 0.05) concentrations. Lake water Fe content was also a good predictor for the long-term accumulation (r(2) = 0.99, P < 0.01), as well as gross sedimentation rate of organic carbon (r(2) = 0.48, P < 0.01).