Unproductive lakes are one of few natural landscape compartments with net release of carbon to the atmosphere. Lakes also generally decrease the net terrestrial carbon uptake, since most of the CO₂ production in unproductive lakes are derived from organic carbon produced on land (e.g. in forests). High latitude lakes are predicted to be particularly affected by the global climate change. The carbon cycling in these lakes and their role in the landscape are therefore important to study.

In this thesis, carbon turnover processes were studied in two lakes above the arctic circle (Lake Diktar-Erik and Lake Merasjärvi) in year 2004 and 2005. Both lakes were net heterotrophic, with large variations in CO₂ concentrations both on shorter (30min) and longer (24h) time-scales. The pelagic habitat supported a major part of the net production of CO₂, with larger dynamics in the CO₂ production than the sediments. The CO₂ variations of the surface water were related to respiration of allochthonous organic carbon, and were affected by the concentration and quality of the DOC, as well as the whole lake water temperatures, and vertical water movements.

The emission of CO₂ from Lake Merasjärvi was measured with the eddy covariance tech­nique. The results showed that the gas transfer rate during moderate winds were higher than expected, causing the two most commonly used models to underestimate the long term fluxes of CO₂ from the lake.

Taken together, the results of the thesis show that the studied lakes contributed to bring terrestrial organic carbon back into the atmosphere, driven by a substantial internal CO₂ production based on mineralization of allochthonous organic carbon. Major results are that the eddy covariance technique indicated that commonly used models tend to underestimate the net release rate of CO₂ from lakes to the atmosphere, and that the lake CO₂ dynamics can be the results of interactions between biogeochemical and physical processes in the lake water.