Hydrological conditions control dissolved organic matter dynamics along a peatland headwater boreal stream

Hydrological conditions (i.e., high-flow versus low-flow) in peatland drainage streams influence both the quantity of dissolved organic carbon (DOC) exports and dissolved organic matter (DOM) composition. Yet, our knowledge on DOM fate after exports from the peatland remains limited while this highly reactive component sustains emissions and exports of carbon dioxide (CO2) from streams through degradation processes. The present study demonstrates the relationships between DOM composition evolution and catchment hydrological conditions along a 3 km long headwater stream running through a boreal peatland, from its source to the outlet. Our results show that hydrological conditions significantly influenced DOM composition evolution along the stream. DOM exported during high-flow conditions presented a composition similar to peat porewater in terms of DOC:DON ratio and aromaticity, but a lower average molecular weight, indicating preferential exports of low molecular weight DOM recently produced in the acrotelm. The DOM composition changed little along the stream during high-flow as it was rapidly flushed downstream. During low-flow conditions, DOM composition evolved along the stream in contrast to high-flow with a strong increase in DOM aromaticity and molecular weight along the stream. These changes were significantly correlated to the water residence time in the stream and to the estimated proportion of mineralized DOC to total DOC flux exported at the stream outlet. These results highlight the importance of hydrological conditions on DOM dynamics as DOM was locally mineralized during low-flow conditions, when DOC exports were low, while mineralization processes happened downstream under high-flow conditions which favored important DOC exports.