Peatland contribution to stream organic carbon exports from a montane watershed

Mountains contain many small and fragmented peatlands within watersheds. As they are difficult to monitor, their role in the water and carbon cycle is often disregarded. This study aimstoassess the streamorganic carbon exports from a montane peatland and characterizesits contribution to the water chemistry in a headstream watershed. High frequency in situmonitoring of turbidity and fDOM were used to quantify respectively particulate (POC) and dissolved (DOC) organic carbon exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting froma few hours to a few days. Forty-sixpercent of the exports occurred during 9% of the time corresponding to the highest flows monitored at the outlet. Despite its small area (3%) within the watershed, the peatland contributes at least63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1±0.4 to 34.6±1.5g.m². yr-1. POC contributes 17% of the total stream organic carbon exports from the watershed. As the frequency of extreme climatic events is expected to increase in thecontext of climate change, further studies should be conducted to understand the evolution of under-estimated mountainous peatland carbon fluxes and their implication in the carbon cycle of headwaters. Plain Language SummarySince the last glacial period, peatlands have accumulated large stocks of organic carbon. Despite representing only 3% of global continental surfaces, they store about 22% of the continental soil carbon stock. In the context of global change,peatlandcarbon sequestration capacity needs to be carefully monitored. In addition to greenhouse gas exchanges with the atmosphere, determining this capacity requires the quantification of aquatic organic carbon exports.Aquatic organic carbon exports have rarely been investigated at mountainous peatlands. Moreover, global change is expected to drastically modify mountain hydrology, influencingaquatic carbon exports and carbon balance of mountainous peatlands. Using high frequency in situinstrumentation, this study shows the annual quantity of aquatic organic carbon exported from a montane peatland in the French Pyrenees is in the samerange asNorthern lowland peatlands.These highly variable exports mainly occur during high discharge events due to snowmelt or rainfalls. Despite its restricted area, this montane peatland is the main contributor of aquatic organic carbon in the watershed. Peatlands influence headwater chemistry and further study must be conducted to monitor the evolution of these mountainous carbon stocks.