Colored organic matter increases CO 2 in meso‐eutrophic lake water through altered light climate and acidity

Many surface waters across the boreal region are browning due to increased concentrations of colored allochthonous dissolved organic carbon (DOC). Browning may stimulate heterotrophic metabolism, may have a shading effect constraining primary production, and may acidify the water leading to decreased pH with a subsequent shift in the carbonate system. All these effects are expected to result in increased lake water carbon dioxide (CO 2 ) concentrations. We tested here these expectations by assessing the effects of both altered allochthonous DOC input and light conditions through shading on lake water CO 2 concentrations. We used two mesocosm experiments with water from the meso‐eutrophic Lake Erken, Sweden, to determine the relative importance of bacterial activities, primary production, and shifts in the carbonate system on CO 2 concentrations. We found that DOC addition and shading resulted in a significant increase in partial pressure of CO 2 ( p CO 2 ) in all mesocosms. Surprisingly, there was no relationship between bacterial activities and p CO 2 . Instead the experimental reduction of light by DOC and/or shading decreased the photosynthesis to respiration ratio leading to increased p CO 2 . Another driving force behind the observed pC O 2 increase was a significant decrease in pH, caused by a decline in photosynthesis and the input of acidic DOC. Considering that colored allochthonous DOC may increase in a warmer and wetter climate, our results could also apply for whole lake ecosystems and p CO 2 may increase in many lakes through a reduction in the rate of photosynthesis and decreased pH.