Photo-chemical reactivity of dissolved organic carbon along a gradient of subarctic lakes with different color

Lake-atmosphere carbon exchanges can be significantly affected by photochemical dissolved organic matter (DOM) mineralization. However, there is an incomplete understanding of how shifting optical characteristics of DOM due to increasing allochthonous organic carbon input affect the photo-reactivity of the DOM per unit of absorbed incoming light. Here, we measured the absorption of ultraviolet (UV) light and subsequent photochemical DOM decay in 148 lakes within the subarctic region of Abisko. These lakes range from brown-water lakes with allochthonous inputs connected to some mires to tundra clear-water lakes with relatively more autochthonous inputs. Fluorescence excitation-emission matrix analysis was used to assess the chemical composition of the DOM. The aim was to see how increasing colored DOM (CDOM) affects the photo-mineralization. We found that the photo decay rates in absolute values were positively correlated to CDOM. However, the photo decay per unit of absorbed light energy did not increase with increasing CDOM; rather, it showed a weak decreasing trend. Fluorescence analyses helped explain these patterns, as humic-like fluorescent DOM of presumable terrestrial origin was associated with high absolute photo decay rates, but not generally with higher photo-reactivity per unit of absorbed light energy than other types of DOM. The results suggest that even though increasing inputs of terrestrial substances mean a higher abundance of photo-degradable materials, it does not necessarily mean that CO₂ emissions increase in lakes where browning limits the ability of light to penetrate deeper water.