| Summary: | Dissolved organic matter (DOM) is the major absorber of sunlight in most natural waters and a critical component of the carbon cycle in aquatic systems. The photochemical decomposition of DOM can have a significant impact on the landscape-atmosphere carbon exchange. However, little is known about how changing characteristics of DOM due to water browning (allochthonous inputs of DOM from terrestrial sources) affect the photochemical processing of the DOM. A shift from in situ self-produced (autochthonous) DOM to allochthonous DOM in clear tundra lakes may imply changes in the optical characteristics and thus photo-reactivity of the DOM. In this study, I measured the absorption of ultraviolet (UV) light and subsequent CO2 emissions resulting from the photochemical DOM decay in 148 subarctic lakes from northern Sweden exposed to artificial sunlight, to assess how increasing chromophoric dissolved organic matter CDOM (browning) may affect the photo mineralization processes. In parallel the chemical composition of the DOM was assessed using fluorescence excitation-emission matrix analysis. The study lakes are situated along an environmental gradient in the subarctic Abisko region, ranging from brown-water lakes connected with some small mires to tundra clear-water lakes. The main objective of this study was to see how increasing CDOM due to increasing terrestrial inputs to the lakes affects the photochemical DOM mineralization. As expected the photo decay rates were positively correlated to the CDOM in absolute values. However, the photo decay per unit of absorbed light energy did not increase with increasing CDOM, but rather showed a weak decreasing trend (R2 = 0.07), which indicates that browning may lead to less photochemically induced CO2 emissions from lakes at any given level of UV light energy absorption. The percentage of lost dissolved organic carbon through photo-mineralization was significantly negatively correlated to the CDOM as well. Fluorescence analyses helped explain these patterns, as humic-like ...
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