Often, accumulation of high concentrations of organic matter only occur in anaerobic habitats because oxygen facilitates rapid degradation rates. However, lakes in the dry Arctic can have dissolved organic carbon (DOC) concentrations that accumulate to >80 mg L-1 (Bogard et al. 2019) while dissolved oxygen concentrations are near saturation levels, so it is unclear why this carbon is not mineralized. Here we combine multi-year measurements of DOC in a range of closed-basin, long retention time lakes, and use optical characterization of DOC quality and experimental treatments to determine the factors that control its accumulation.
Results/Conclusions
Although some seasonal variability in DOC concentration occurs, optical measures of DOC quality suggested that there was input and accumulation of low molecular weight DOC that had been degraded by sunlight. We found a strong relationship between the 14C-age of DOM and lake water specific conductivity, with mean ages varying from modern in the freshest lakes to ca. 1000 years in lakes with the highest conductivity (ca. 5000 µS cm-1). Aging corresponded with increased DOC concentration, increased δ13C-DOC isotope ratios, and decreased spectral slopes and SUVA values, suggesting photochemical and microbial degradation played important roles in DOM processing. Furthermore, the degradation of young DOM was strongly stimulated by inorganic P, but P was less stimulatory to older, more recalcitrant DOM. Together these results indicate that photochemical and microbial processing of DOM under P-limitation in arid environments can lead to large pools of recalcitrant organic matter with low degradation rates, despite presence of oxygen. These results mean that when and where organic matter accumulates on the Earth can be strongly linked to regional climate. Especially in arid environments when oxygen is present our results point to future stores of recalcitrant DOM as a fate of carbon in a changing Arctic.