PS 36-1 - Water browning effect on the photo-reactivity of the dissolved organic carbon in subarctic lakes in northern Sweden

Abstract: Photochemical decomposition of dissolved organic matter (DOM) can significantly impact the landscape-atmosphere carbon exchange. However, little is known about how changing characteristics of DOM due to water browning (increased inputs of colored DOM [CDOM] from terrestrial sources) affect the photochemical processing of DOM. In this study, we measured the absorption of ultraviolet light and the subsequent photochemical CO₂ production in 148 arctic lakes from northern Sweden. Samples were exposed to artificial sunlight, to assess how the increasing CDOM may affect the photo mineralization processes. The photo decay rates in carbon units were positively correlated to the CDOM. However, the photo decay per unit of absorbed light energy did not increase with increasing CDOM. Fluorescence analyses helped explain these patterns, as humic-like fluorescent DOM of presumable terrestrial origin was generally not more photo-reactive than other types of DOM. The results suggest that even though increasing inputs of CDOM means higher abundance of photo degradable materials, it would not mean that the CO₂ emissions increase in lakes where browning limits the ability of light to penetrate deeper water.