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 aims to assess the stream organic carbon exports from a montane peatland and characterizes its contribution to the water chemistry in a headstream watershed. High frequency in situ monitoring of turbidity and fDOM were used to quantify respectively particulate organic carbon (POC) and dissolved organic carbon (DOC) exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1,343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting from a few hours to a few days. Forty-six percent 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 least 63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1 ± 0.4 to 34.6 ± 1.5 g m 2 year −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 the context of climate change, further studies should be conducted to understand the evolution of underestimated 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, peatland carbon 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, influencing aquatic carbon exports and carbon balance of mountainous peatlands. Using high frequency in situ instrumentation, this study shows the annual quantity of aquatic organic carbon exported from a montane peatland in the French Pyrenees is in the same range as Northern 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.

Peatlands are key components of the global carbon cycle. We compiled peatland dissolved organic carbon (DOC) export from 62 sites to provide a global estimate of the contribution of these ecosystems to fluvial organic carbon fluxes. Despite their modest coverage on land (less than 3%), peatlands contribute at least 91 ± 54 Tg DOC to surface waters, and could represent from 12% to 20% of the DOC ultimately transferred to oceans. Boreal peatlands are the main contributors, with 58% of peatland inputs. Tropical peatlands contribute 37% of global peatland fluxes although representing only 13% of global peatland areas. Temperature is the leading factor explaining the variability in DOC fluxes for natural sites, when it is discharge for sites impacted by direct anthropogenic activities. This suggests different trajectories under climate change pressure for non–disturbed and disturbed sites and reinforces the need for hydrological restoration of disturbed peatland sites worldwide.

Abstract. Peatlands store ∼ 20 %–30 % of the global soil organic carbon stock and are an important source of dissolved organic carbon (DOC) for inland waters. Recent improvements for in situ optical monitoring revealed that the DOC concentration in streams draining peatlands is highly variable, showing seasonal variation and short and intense DOC concentration peaks. This study aimed to statistically determine the variables driving stream DOC concentration variations at seasonal and event scales. Two mountainous peatlands (one fen and one bog) were monitored in the French Pyrenees to capture their outlet DOC concentration variability at a high-frequency rate (30 min). Abiotic variables including precipitation, stream temperature and water level, water table depth, and peat water temperature were also monitored at high frequency and used as potential predictors to explain DOC concentration variability. Results show that at both sites DOC concentration time series can be decomposed into a seasonal baseline interrupted by many short and intense peaks of higher concentrations. The DOC concentration baseline is driven, at the seasonal scale, by peat water temperature. At the event scale, DOC concentration increases are mostly driven by a rise in the water table within the peat at both sites. Univariate linear models between DOC concentration and peat water temperature or water table increases show greater efficiency at the fen site. Water recession times were derived from water level time series using master recession curve coefficients. They vary greatly between the two sites but also within one peatland site. They partly explain the differences between DOC dynamics in the studied peatlands, including peat porewater DOC concentrations and the links between stream DOC concentration and water table rise within the peatlands. This highlights that peatland complexes are composed of a mosaic of heterogeneous peat units distinctively producing or transferring DOC to streams.