Abstract
AbstractWhen organic matter from thawed permafrost is released, the sources and sinks of greenhouse gases (GHGs), like carbon dioxide (CO2) and methane (CH4) in Arctic rivers will be influenced in the future. However, the temporal variation, environmental controls, and magnitude of the Arctic riverine GHGs are largely unknown. We measured in situ high temporal resolution concentrations of CO2, CH4, and oxygen (O2) in the Ambolikha River in northeast Siberia between late June and early August 2019. During this period, the largely supersaturated riverine CO2 and CH4 concentrations decreased steadily by 90% and 78%, respectively, while the O2 concentrations increased by 22% and were driven by the decreasing water temperature. Estimated gas fluxes indicate that during late June 2019, significant emissions of CO2 and CH4 were sustained, possibly by external terrestrial sources during flooding, or due to lateral exchange with gas‐rich downstream‐flowing water. In July and early August, the river reversed its flow constantly and limited the water exchange at the site. The composition of dissolved organic matter and microbial communities analyzed in discrete samples also revealed a temporal shift. Furthermore, the cumulative total riverine CO2 emissions (36.8 gC‐CO2 m−2) were nearly five times lower than the CO2 uptake at the adjacent floodplain. Emissions of riverine CH4 (0.21 gC‐CH4 m−2) were 16 times lower than the floodplain CH4 emissions. Our study revealed that the hydraulic connectivity with the land in the late freshet, and reversing flow directions in Arctic streams in summer, regulate riverine carbon replenishment and emissions.
Highlights
Rivers connect the land with oceans and are effective active pipes where terrestrial carbon undergoes numerous transformations (Cole et al, 2007)
The water discharge (Q) measured at the Ambolikha River Site (ARS) (Figure 1) portrayed a decrease from the late freshet to summer (Figure 2a), which is in agreement with the decreasing phase of the peak freshet discharge in the Kolyma River (Figure S1 in Supporting Information S1) (Shiklomanov et al, 2020)
The calculated water retention time indicated that at the start of Phase 1 the average time it took for a water parcel to travel downstream from the ARS to point 3 was about 1 day
Summary
Rivers connect the land with oceans and are effective active pipes where terrestrial carbon undergoes numerous transformations (Cole et al, 2007). The majority of the carbon in rivers and streams is emitted into the atmosphere as carbon dioxide (CO2) (Raymond et al, 2013) and methane (CH4) (Stanley et al, 2016). It is estimated that less than 50% (∼0.9 PgC yr−1) of the total carbon export from global inland waters (1.9–3.2 PgC yr−1) reaches the oceans (Abril & Borges, 2019). Castro-Morales Writing – review & editing: K.
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