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Abstract
Abstract. Reservoir sediments sequester significant amounts of organic carbon (OC), but at the same time, high amounts of methane (CH4) can be produced and emitted during the degradation of sediment OC. While the greenhouse gas emission of reservoirs has received a lot of attention, there is a lack of studies focusing on OC burial. In particular, there are no studies on reservoir OC burial in the Amazon, even though hydropower is expanding in the basin. Here we present results from the first investigation of OC burial and CH4 concentrations in the sediments of an Amazonian hydroelectric reservoir. We performed sub-bottom profiling, sediment coring and sediment pore water analysis in the Curuá Una (CUN) reservoir (Amazon, Brazil) during rising- and falling-water periods. The spatially resolved average sediment accumulation rate was 0.6 cm yr−1, and the average OC burial rate was 91 g C m−2 yr−1. This is the highest OC burial rate on record for low-latitude hydroelectric reservoirs. Such a high rate probably results from a high OC deposition onto the sediment, which compensates the high OC mineralization at a 28–30 ∘C water temperature. Elevated OC burial was found near the dam and close to major river inflow areas. C:N ratios between 10.3 and 17 (average ± SD: 12.9±2.1) suggest that both land-derived and aquatic OC accumulate in CUN sediments. About 23 % of the sediment pore water samples had dissolved CH4 above the saturation concentration. This represents a higher share than in other hydroelectric reservoirs, indicating a high potential for CH4 ebullition, particularly in river inflow areas.
Highlights
Freshwater ecosystems represent a small fraction of the global area (∼ 4 % of terrestrial area; Downing et al, 2012; Verpoorter et al, 2014), they play an important role in the global carbon cycle, both emitting carbon to the atmosphere and burying carbon in the sediments (Cole et al, 2007; Tranvik et al, 2009)
The highest values of organic carbon (OC) burial were observed near the dam, at the confluence of the major inflowing rivers and in the inflow area of the main tributary of the Curuá Una River (Fig. 1)
Our sampling was representative of the whole system, from the margins, where there is a greater presence of dead tree trunks, to the riverbed, where the sedimentation was lower (Fig. 1)
Summary
Freshwater ecosystems represent a small fraction of the global area (∼ 4 % of terrestrial area; Downing et al, 2012; Verpoorter et al, 2014), they play an important role in the global carbon cycle, both emitting carbon to the atmosphere and burying carbon in the sediments (Cole et al, 2007; Tranvik et al, 2009). Since a part of the buried OC may offset a share of greenhouse gas emission, it is essential to include OC burial in the carbon balance of inland water ecosystems (Kortelainen et al, 2013; Mendonça et al, 2017). Considering the importance of reservoirs as a carbon sink (∼ 28 % to 55 % of total inland water OC burial; Mendonça et al, 2017) and the increasing number of hydroelectric dams (Zarfl et al, 2015), the limited number of studies on OC burial in reservoirs severely hampers the understanding of this important
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