Sources and controls of organic carbon in lakes across the subarctic treeline

Abundant northern lakes have an intrinsic role in the transport, sequestration, and mineralization of terrestrial organic carbon. The quantity and quality of this carbon control vital aquatic biogeochemical processes, and influence the metabolic balance of lakes with subsequent impact on the global carbon cycle. We measured concentrations and type of dissolved organic matter and elemental and stable isotopic composition of carbon and nitrogen in 31 subarctic lakes with varying catchment types across the treeline in northern Finland, integrating both the pelagic (lake water) and the benthic (surface sediments) carbon pools for a comprehensive understanding of landscape influence on aquatic carbon dynamics. Wetland cover was identified as the primary catchment control over the aquatic carbon pools, reflected particularly in the bio-optical properties of lake water. Landscape influence on sediment carbon content and composition, mirroring largely the structure and productivity of the aquatic communities, was primarily connected to allochthonous nutrient inputs fueling autotrophic production. Basin depth and benthic production were identified as important internal controls on the surface sediment geochemistry. Overall, our results suggest that shallow subarctic lakes will be particularly susceptible to climate-mediated changes in the export of terrestrial organic matter from wetlands. Whether the landscape influence will promote the channeling of terrestrial carbon into the atmosphere via aquatic ecosystems will strongly depend on the interplay between the biogeochemical characteristic of the allochthonous carbon inputs, terrestrial nutrient fluxes, and the depth of the recipient ecosystems.