Peatland Contribution to Stream Organic Carbon Exports From a Montane Watershed

Abstract

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.

Author’s response to reviewer 1

“…How water temperature is related with the input and output of organic carbon in the bog and the fen? We agree that concentration of DOC shows the dynamic balance between the input and output of organic carbon; However in these mountainous peatlands we observed that DOC concentrations are really lower at the inlet than at the outlet, as mentioned by Rosset et al, (2019). Thus, the mechanisms which control DOC concentration at the outlet occurs mainly within these peatlands and we did not consider that input of organic carbon from the inlet was a valuable variable to investigate, as input water temperature.…”

“…At both sites watershed boundaries have been delineated using DEM models, however only the peatland areas (3% of the watershed area in Bernadouze and 6% in Ech) were delineated on the figure 1 (orange lines). Peatlands are the main contributors of DOC at the outlets as reported in Rosset et al, 2019. This was explicitly written in the manuscript P4 L 6 Fig.…”

Drivers of seasonal and event scale DOC dynamics at the outlet of mountainous peatlands revealed by high frequency monitoring

“…Different drivers have been identified depending on the latitude of the studied peatland sites and the time scale considered. DOC concentrations were found to be negatively correlated with discharge in boreal systems (Köhler et al, 2008), positively correlated with discharge in temperate areas (Clark et al, 2007) or non-correlated with discharge in mountainous areas (Rosset et al, 2019). Temperature was also reported as an important driver of seasonal variations of DOC concentration in field (Billett et al, 2006) and mesocosm (Pastor et al, 2003) experiments since DOC production is boosted by a greater microbial activity during warmer periods.…”

“…Mountains host many small peatland areas that are often neglected in global peatland assessments but which drastically influence stream chemistry in headwater catchments (Rosset et al, 2019).The harsh mountainous climatic conditions (from the montane to the alpine belt (Holdridge, 1967)) and the relief of those areas generate high gradients of different abiotic parameters (temperature, precipitation, hydrology). Furthermore, seasonal climatic conditions are contrasted, making it possible to differentiate seasonal from event scale stream DOC concentration variability.…”

Drivers of seasonal- and event-scale DOC dynamics at the outlet of mountainous peatlands revealed by high-frequency monitoring

“…Mountains host many small peatland areas that are often neglected in global peatland assessments but which drastically influence stream chemistry in headwater catchments (Broder and Biester, 2015;Rosset et al, 2019). The harsh mountainous climatic conditions (from the montane to the alpine belt, Holdridge et al, 1967) and the relief of those areas generate high gradients of different abiotic parameters (temperature, precipitation, hydrology) evolving along both seasonal and event (snowmelt, rainstorms) scales.…”