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 (CO 2 ) 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 CO 2 to the atmosphere from the three small uppermost lakes, whereas in two large lowland lakes the CO 2 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 CO 2 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).