Carbon dynamics in lakes of the boreal forest under a changing climate

Water-covered lands comprise approximately 30% of the total area of the world's boreal forest biome. Most of these lands are peatlands (i.e., bogs and fens), which store over half of the total carbon in the biome. Because climate warming threatens to alter the carbon stocks of peatlands, much attention has been devoted to understanding the climatic and hydrologic conditions that affect peatland biogeochemistry. However, there are other aquatic systems that are widespread in the boreal forest that also process and store carbon, including lakes and ponds. Although non-peatland aquatic systems cover a much smaller portion of the boreal landscape, they still contain approximately 15% of the total carbon pool for the biome, much of it stored as either profundal or littoral sediments. Further, the carbon dynamics of boreal lakes are dynamically coupled to watershed processes. Excepting major disturbances to boreal catchments, such as forest fires and forest harvest, surface waters are the only locations of net loss of carbon to the atmosphere. Our objectives are to review what is known about factors that affect lake ecosystem carbon dynamics in the boreal forest and to identify areas of study that we deem to be profitable for forecasting the impacts of climate change on carbon pools and flux rates. We primarily focus on the boreal forest of North America, but recognize that our findings may also be relevant for boreal areas of Fennoscandia and Russia. The following research priorities are identified: (i) estimation of carbon pools in profundal and littoral sediments across the boreal forest, (ii) warming experiments that include quantification of ecosystem carbon dynamics in addition to measuring changes to aquatic food web structure, (iii) whole system experiments to understand the hydrologic and biogeochemical conditions by which allochthonous carbon is integrated into aquatic food webs, especially in the context of increased nutrient concentrations associated with a warmer, and possibly drier, climate, as forecast for the southern boreal forest, (iv) watershed-scale assessment of carbon budgets for lakes that straddle transitional zones between the boreal forest and prairie-parkland, temperate forest or tundra, to detect evidence of ecosystem migration, and (v) integration of lacustrine carbon pools and flux rates into carbon budgets at scales that range from local watersheds to the boreal forest biome.