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The origin and fate of sediment organic carbon in tropical reservoirs
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0002-4634-527x
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
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 [en]
methane, carbon burial, tropical reservoirs, sediment, carbon cycle, limnology
National Category
Geosciences, Multidisciplinary Environmental Sciences
Research subject
Biology with specialization in Limnology
Identifiers
URN: urn:nbn:se:uu:diva-408492ISBN: 978-91-513-0938-5 (print)OAI: oai:DiVA.org:uu-408492DiVA, id: diva2:1422476
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, 336642Available from: 2020-05-13 Created: 2020-04-07 Last updated: 2020-05-13
List of papers
1. Reduced Mineralization of Terrestrial OC in Anoxic Sediment Suggests Enhanced Burial Efficiency in Reservoirs Compared to Other Depositional Environments
Open this publication in new window or tab >>Reduced Mineralization of Terrestrial OC in Anoxic Sediment Suggests Enhanced Burial Efficiency in Reservoirs Compared to Other Depositional Environments
2019 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 124, no 3, p. 678-688Article in journal (Refereed) Published
Abstract [en]

Freshwater reservoirs are important sites of organic carbon (OC) burial, but the extent to which reservoir OC burial is a new anthropogenic carbon sink is currently unclear. While burial of aquatic OC (by, e.g., phytoplankton) in reservoirs may count as a new C sink, the burial of terrestrial OC in reservoirs constitutes a new C sink only if the burial is more efficient in reservoirs than in other depositional environments. We carried out incubation experiments that mimicked the environmental conditions of different depositional environments along the land‐sea continuum (oxic and anoxic freshwater, oxic and anoxic seawater, oxic river bedload, and atmosphere‐exposed floodplain) to investigate whether reservoirs bury OC more efficiently compared to other depositional environments. For sediment OC predominantly of terrestrial origin, OC degradation rates were significantly lower, by a factor of 2, at anoxic freshwater and saltwater conditions compared to oxic freshwater and saltwater, river, and floodplain conditions. However, the transformation of predominantly terrestrial OC to methane was one order of magnitude higher in anoxic freshwater than at other conditions. For sediment OC predominantly of aquatic origin, OC degradation rates were uniformly high at all conditions, implying equally low burial efficiency of aquatic OC (76% C loss in 57 days). Since anoxia is more common in reservoirs than in the coastal ocean, these results suggest that reservoirs are a depositional environment in which terrestrial OC is prone to become buried at higher efficiency than in the ocean but where also the terrestrial OC most efficiently is transformed to methane.

Abstract [en]

Plain Language Summary

The widespread construction of dams disrupts the transport of sediment particles by rivers and traps sediment in reservoirs. The sediment contains organic carbon (OC) that can be degraded into greenhouse gas (carbon dioxide and methane) or buried in the sediment. In the absence of dams, sediment would be deposited in other environments, such as rivers, floodplains, or sea. In our study, we investigated through incubation experiments if the construction of reservoirs creates an environment that stores sediment OC more efficiently than other environments. We found that OC from terrestrial origin is buried more efficiently in anoxic conditions than in oxic conditions. Slowing down of water and high primary production makes reservoirs prone to anoxia, implying that they may be sites of highly efficient OC burial. However, anoxic reservoir conditions were also characterized by higher methane emissions than other environments. OC from aquatic origin was degraded extensively in all studied conditions. Our results suggest that the ~50% of the terrestrial OC that escapes degradation if it gets trapped in an anoxic reservoir may be accounted as a new carbon sink; however, whole‐system studies that account for site‐specific environmental conditions are necessary to calculate the magnitude of this effect.

Keywords
sediment, burial, carbon, anoxic, reservoir, methane
National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-375404 (URN)10.1029/2018JG004823 (DOI)000464653200016 ()
Funder
EU, FP7, Seventh Framework Programme, 336642
Available from: 2019-01-30 Created: 2019-01-30 Last updated: 2020-04-07Bibliographically approved
2. Methane formation in tropical reservoirs predicted from sediment age and nitrogen
Open this publication in new window or tab >>Methane formation in tropical reservoirs predicted from sediment age and nitrogen
2019 (English)In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 9, article id 11017Article in journal (Refereed) Published
Abstract [en]

Freshwater reservoirs, in particular tropical ones, are an important source of methane (CH4) to the atmosphere, but current estimates are uncertain. The CH4 emitted from reservoirs is microbially produced in their sediments, but at present, the rate of CH4 formation in reservoir sediments cannot be predicted from sediment characteristics, limiting our understanding of reservoir CH4 emission. Here we show through a long-term incubation experiment that the CH4 formation rate in sediments of widely different tropical reservoirs can be predicted from sediment age and total nitrogen concentration. CH4 formation occurs predominantly in sediment layers younger than 6-12 years and beyond these layers sediment organic carbon may be considered effectively buried. Hence mitigating reservoir CH4 emission via improving nutrient management and thus reducing organic matter supply to sediments is within reach. Our model of sediment CH4 formation represents a first step towards constraining reservoir CH4 emission from sediment characteristics.

National Category
Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-387547 (URN)10.1038/s41598-019-47346-7 (DOI)000477702400001 ()31358820 (PubMedID)
Funder
EU, FP7, Seventh Framework Programme, 336642
Available from: 2019-06-24 Created: 2019-06-24 Last updated: 2020-04-07Bibliographically approved
3. Contribution of aquatic and terrestrial sources to sediment organic carbon of four contrasting tropical reservoirs.
Open this publication in new window or tab >>Contribution of aquatic and terrestrial sources to sediment organic carbon of four contrasting tropical reservoirs.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Following construction of a dam, a large amount of sediment is accumulated in the reservoir. The sediment contains organic carbon (OC) that is produced within the reservoir (aquatic) or on land (terrestrial). The origin of the sediment OC can influence the fate of the OC (burial or mineralization) as well as the extent to which the accumulating sediment OC may be accounted as a new carbon sink. In spite of its importance for evaluation of the carbon footprint of reservoirs, however, studies on the sediment OC origin in reservoirs are scarce.

We therefore estimated sources of OC to four contrasting tropical reservoir sediments using lipid biomarkers (n-alkanes). The reservoirs ranged in their trophic status from oligotrophic to eutrophic. We used surface sediment to estimate the source of OC to reservoir sediment, and deep sediment to estimate the source of buried OC.

We found that terrestrial material (terrestrial plants and soil) was the largest source of sediment OC (61%; a mean of all samples). Terrestrial OC was dominant in river inflows, while aquatic OC increased towards dams. Particularly high aquatic OC in sediment (up to 92%) was found at locations of cyanobacteria blooms. The fraction of aquatic OC in surface sediment increased with reservoir productivity from 31% in oligotrophic reservoir, 45% in mesotrophic, 58% in meso-eutrophic to 66% in eutrophic reservoir. The fraction of aquatic OC was generally lower in deep sediment (33%; a mean of all deep) than in surface sediment (50%). This aquatic OC in deep sediment has a potential to be buried and constitutes an anthropogenic OC sink, thereby decreasing hydropower carbon footprint.

Keywords
sediment, carbon, aquatic, terrestrial
National Category
Geosciences, Multidisciplinary
Research subject
Biology with specialization in Limnology
Identifiers
urn:nbn:se:uu:diva-408490 (URN)
Funder
EU, FP7, Seventh Framework Programme
Available from: 2020-04-07 Created: 2020-04-07 Last updated: 2020-04-07
4. High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir
Open this publication in new window or tab >>High organic carbon burial but high potential for methane ebullition in the sediments of an Amazonian hydroelectric reservoir
Show others...
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.

National Category
Oceanography, Hydrology and Water Resources
Identifiers
urn:nbn:se:uu:diva-408491 (URN)10.5194/bg-17-1495-2020 (DOI)000522152700001 ()
Available from: 2020-04-07 Created: 2020-04-07 Last updated: 2020-05-06Bibliographically approved

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