Photochemical reactivity of dissolved organic carbon in subarctic lakes along a color gradient

Abstract

Lake–atmosphere carbon exchanges can be significantly affected by photochemical dissolved organic matter (DOM) mineralization. However, our understanding of how increasing allochthonous organic carbon input affects the photoreactivity of DOM per unit of absorbed incoming light is incomplete. Here, we measured the absorption of ultraviolet (UV) light and subsequent photochemical DOM decay in 148 lakes within the subarctic region of Abisko, Sweden. These lakes range from brown-water lakes with allochthonous input from mires to tundra clear-water lakes with relatively more autochthonous input. We used fluorescence excitation–emission matrix analysis to assess the DOM chemical composition to determine how increasing colored DOM (CDOM) affects photomineralization. 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; humic-like fluorescent DOM, presumably of terrestrial origin, was associated with high absolute photo decay rates, but not generally with higher photoreactivity per unit of absorbed light energy than other types of DOM. The results suggest that even though increasing inputs of terrestrial substances lead to a higher abundance of photodegradable materials, CO2 emissions do not necessarily increase in lakes where browning limits the ability of light to penetrate deeper water.