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High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir
Laboratório de Ecologia Aquática, Programa de Pós-Graduação em Ecologia, Universidade Federal de Juiz de Fora, 36036 900, Juiz de Fora, Brazil.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0003-1351-9277
Laboratório de Ecologia Aquática, Programa de Pós-Graduação em Ecologia, Universidade Federal de Juiz de Fora, 36036 900, Juiz de Fora, Brazil.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0002-4634-527x
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2020 (English)In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 17, no 6, p. 1495-1505Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2020. Vol. 17, no 6, p. 1495-1505
National Category
Oceanography, Hydrology and Water Resources
Identifiers
URN: urn:nbn:se:uu:diva-408491DOI: 10.5194/bg-17-1495-2020ISI: 000522152700001OAI: oai:DiVA.org:uu-408491DiVA, id: diva2:1422334
Available from: 2020-04-07 Created: 2020-04-07 Last updated: 2020-06-23Bibliographically approved
In thesis
1. The origin and fate of sediment organic carbon in tropical reservoirs
Open this publication in new window or tab >>The origin and fate of sediment organic carbon in tropical reservoirs
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Recently, the construction of reservoirs has boomed, particularly in the tropics, but the impact of reservoirs on the global carbon cycle is not evident. Reservoirs accumulate sediments that simultaneously bury organic carbon (OC) and thus act as a C sink, and also produce methane (CH4) and thus emit a strong greenhouse gas. High temperature, internal production and sedimentation rates in tropical reservoirs may enhance both OC burial and CH4 production, however to a currently unknown extent. This thesis investigates the efficiency of the OC sink as well as the OC sources that feed into OC burial and CH4 production in four contrasting tropical reservoirs in Brazil. 

The results demonstrate that reservoir sediments receive both terrestrial and aquatic OC, and that terrestrial OC is more prevalent in reservoirs with low internal production, and in river inflow bays. Aquatic OC is present in the sediments of all studied reservoirs, particularly in the reservoirs with high internal production and at sites that are closer to the dam. Reservoirs that experience anoxic conditions or high sediment deposition rates are likely to bury terrestrial OC at higher efficiency than oxic environments, such as oxygenated reservoirs, rivers, floodplains and sea, while aquatic OC degrades as similar rates in both oxic and anoxic environments. Deposition of OC in anoxic sediment, however, results in high CH4formation rates that strongly depend on sediment age and nitrogen content. The CH4 formation decreases exponentially with sediment age, but never ceases completely in the studied reservoir sediment. CH4 formation is highest but decreases more rapidly over time in sediment with a high share of nitrogen-rich aquatic OC, indicating that management of nutrient input into the reservoir may decrease sediment CH4 formation.

The thesis illustrates that reservoir sediments bury aquatic OC and also bury terrestrial OC with high efficiency, which represents an anthropogenic carbon sink that decreases the carbon footprint of hydropower. Simultaneously, the reservoir sediment produces CH4 that may be emitted into the atmosphere and consequently elevates the carbon footprint of hydropower. However, reservoir CH4 emission may be mitigated by reducing nutrient input into rivers and reservoirs.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2020. p. 47
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1930
Keywords
methane, carbon burial, tropical reservoirs, sediment, carbon cycle, limnology
National Category
Geosciences, Multidisciplinary Environmental Sciences
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-408492 (URN)978-91-513-0938-5 (ISBN)
Public defence
2020-06-04, Ekmansalen, EBC (Evolutionary Biology Centre), Norbyvägen 18, Uppsala, 13:00 (English)
Opponent
Supervisors
Projects
HYDROCARB
Funder
EU, FP7, Seventh Framework Programme, 336642
Available from: 2020-05-13 Created: 2020-04-07 Last updated: 2020-06-17

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Sobek, SebastianIsidorova, Anastasija

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