• carbon;
  • nitrogen;
  • boreal lakes;
  • C evasion/accumulation;
  • drivers;
  • water chemistry

[1] The role of lakes in landscape carbon (C) cycling and primary drivers behind freshwater C balance have remained poorly known, although lakes are an important landscape component and cover 10% of Finland's total area. We studied CO2 evasion and average Holocene C accumulation in 82 boreal lakes (0.04–1540 km2; max depth 1–93 m) located between the latitudes 60°N and 69°N. Both CO2 evasion and C accumulation correlated with numerous drivers and were closely linked to lake area and maximum depth. The total aquatic C retention (C evasion + C accumulation) was largely determined by lake area (r2 = 0.96, p < 0.0001). Carbon in boreal lakes is mostly terrestrially fixed and our data demonstrate that boreal lakes are important conduits for transferring terrestrially fixed C to the atmosphere. C evasion/accumulation ratio (Cev/Cacc) ranged from 4 to 86 (on average 30) and correlated positively only with lake water NO3-N concentration and maximum depth, and negatively with sediment N pool. Numerous possible important drivers like land use and climate as well as lake physical and chemical characteristics were compared to C balances. Our study might thus be valuable in trying to understand links between aquatic and terrestrial C cycling over boreal landscapes. Although climatic drivers generally play a key role in short-term C balance fluctuations, our data indicate nitrogen/fertility to be a key contributing factor to long term C balance—increasing Cev/Cacc and thus decreasing the role of lakes in landscape C sequestration in boreal zone.