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1. How green can Amazon hydropower be? Net carbon emission from the largest hydropower plant in Amazonia

   https://doi.org/10.1126/sciadv.abe1470  

   Bertassoli, Dailson J.; Sawakuchi, Henrique O.; de Araújo, Kleiton R.; de Camargo, Marcelo G.; Alem, Victor A.; Pereira, Tatiana S.; Krusche, Alex V.; Bastviken, David; Richey, Jeffrey E.; Sawakuchi, André O. (June 2021, Science Advances)

   The current resurgence of hydropower expansion toward tropical areas has been largely based on run-of-the-river (ROR) dams, which are claimed to have lower environmental impacts due to their smaller reservoirs. The Belo Monte dam was built in Eastern Amazonia and holds the largest installed capacity among ROR power plants worldwide. Here, we show that postdamming greenhouse gas (GHG) emissions in the Belo Monte area are up to three times higher than preimpoundment fluxes and equivalent to about 15 to 55 kg CO 2 eq MWh −1 . Since per-area emissions in Amazonian reservoirs are significantly higher than global averages, reducing flooded areas and prioritizing the power density of hydropower plants seem to effectively reduce their carbon footprints. Nevertheless, total GHG emissions are substantial even from this leading-edge ROR power plant. This argues in favor of avoiding hydropower expansion in Amazonia regardless of the reservoir type. 

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2. Low Diffusive Methane Emissions From the Main Channel of a Large Amazonian Run-of-the-River Reservoir Attributed to High Methane Oxidation

   https://doi.org/10.3389/fenvs.2021.655455  

   Sawakuchi, Henrique O.; Bastviken, David; Enrich-Prast, Alex; Ward, Nicholas D.; Camargo, Plínio B.; Richey, Jeffrey E. (April 2021, Frontiers in Environmental Science)

   The global development of hydropower dams has rapidly expanded over the last several decades and has spread to historically non-impounded systems such as the Amazon River’s main low land tributaries in Brazil. Despite the recognized significance of reservoirs to the global methane (CH 4 ) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH 4 dynamics in the main channel and downstream of the Santo Antônio hydroelectric reservoir, a large tropical run-of-the-river (ROR) reservoir in Amazonia. This study is intended to give a snapshot of the CH 4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH 4 production in reservoirs’ littoral sediment than in the naturally flooded areas downstream of the dam. Despite the large production in the reservoir or naturally flooded areas, high CH 4 oxidation in the main channel keep the concentration and fluxes of CH 4 in the main channel low. Similar CH 4 concentrations in the reservoir and downstream close to the dam suggest negligible degassing at the dam, but stable isotopic evidence indicates the presence of a less oxidized pool of CH 4 after the dam. ROR reservoirs are designed to disturb the natural river flow dynamics less than traditional reservoirs. If enough mixing and oxygenation remain throughout the reservoir’s water column, naturally high CH 4 oxidation rates can also remain and limit the diffusive CH 4 emissions from the main channel. Nevertheless, it is important to highlight that our results focused on emissions in the deep and oxygenated main channel. High emissions, mainly through ebullition, may occur in the vast and shallow areas represented by bays and tributaries. However, detailed assessments are still required to understand the impacts of this reservoir on the annual emissions of CH 4 . 

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3. Seasonal and spatial variability of CO2 in aquatic environments of the central lowland Amazon basin.

   https://doi.org/https://doi.org/10.1007/s10533-019-00554-9(01234567.  

   Amaral, J.H.F.; Farjalla, V.F.; Melack, J.M.; Kasper, D.; Scofield, V.; Barbosa, P.M.; Forsberg, B.R. (February 2019, Biogeochemistry)

   Different sources and processes contribute to pCO2 and CO2 exchange with the atmosphere in the rivers and floodplains of the Amazon basin. We measured or estimated pCO2, CO2 fluxes with the atmosphere, planktonic community respiration (PCR), and environmental and landscape variables along the Negro and Amazon-Solimões rivers during different periods of the fluvial hydrological cycle. Values of pCO2 ranged from 307 to 7,527 μatm, while CO2 fluxes ranged from -9.3 to 1,128 mmol m-2 d-1 in the Amazon-Solimões basin. In the Negro basin, pCO2 values ranged from 648 to 6,526 μatm, and CO2 fluxes from 35 to 1,025 mmol m-2 d-1. In a general linear model including data from Negro and Amazon-Solimões basins, seasonal and spatial variation in flooded vegetated habitat area, dissolved oxygen, depth and water temperature explained 85% of surface pCO2 variation. Levels of pCO2 varied with inundation extent, with higher pCO2 values occurring in periods with greater water depth and inundation area, and lower dissolved oxygen concentrations and water temperatures. In a separate analysis for the Amazon-Solimões river and floodplains, ecosystem type (lotic or lentic), hydrological period, water temperature, dissolved oxygen, depth and dissolved phosphorus explained 83% of pCO2 variation. Our results demonstrate the influence of alluvial floodplains and seasonal variations in their limnological characteristics on the pCO2 levels in river channels of the lowland Amazon. 

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4. The Abundance, Size, and Spacing of Lakes and Reservoirs Connected to River Networks

   https://doi.org/10.1029/2018GL080841  

   Gardner, J. R.; Pavelsky, T. M.; Doyle, M. W. (March 2019, Geophysical Research Letters)

   Abstract Descriptions of river network topology do not include lakes/reservoirs that are connected to rivers. We describe the properties and scaling patterns of river network topology across the contiguous United States: how lake/reservoir abundance, median lake/reservoir size, and median lake/reservoir spacing change with river size. Typically, lake/reservoir abundance decreases, median lake/reservoir size increases, but median lake/reservoir spacing is uniform across river size. There is a characteristic lake/reservoir size of 0.01–0.05 km²and a characteristic lake/reservoir spacing of 1–5 km that shifts to 27–61 km in larger rivers. Climate explains more of the variance in river network topology than both glacial history and constructed reservoirs. Our results provide conceptual models for building river network topologies to assess how lake/reservoir abundance, size, and spacing effect the transport, storage, and cycling of water, materials, and organisms across networks. 

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5. Magnitudes and Drivers of Greenhouse Gas Fluxes in Floodplain Ponds During Drawdown and Inundation by the Three Gorges Reservoir

   https://doi.org/10.1029/2018JG004701  

   Miller, B. L.; Chen, H.; He, Y.; Yuan, X.; Holtgrieve, G. W. (August 2019, Journal of Geophysical Research: Biogeosciences)

   Abstract Hydropower reservoirs are well‐known emitters of greenhouse gases to the atmosphere. This is due in part to seasonal water level fluctuations that transfer terrestrial C and N from floodplains to reservoirs. Partial pressures and fluxes of the greenhouse gases CH₄, CO₂, and N₂O are also a function of in situ biological C and N cycling and overall ecosystem metabolism, which varies on a diel basis within inland waters. Thus, greenhouse gas emissions in hydropower reservoirs likely vary over seasonal and diel time scales with local hydrology and ecosystem metabolism. China's Three Gorges Reservoir is among the largest and newest in the world, with a floodplain that encompasses approximately one third of the reservoir area. We measured diel partial pressures and fluxes of greenhouse gases in ponds on the Three Gorges Floodplain. We repeated these measurements on the submerged floodplain following inundation by the Three Gorges Reservoir. During reservoir drawdown, CH₄ebullition comprised 60–68% of emissions from floodplain ponds to the atmosphere. Using linear mixed effects modeling, we show that partial pressures of CH₄and CO₂and diffusive CO₂fluxes in floodplain ponds varied on a diel basis with in situ respiration. Floodplain inundation by the Three Gorges Reservoir significantly moderated areal CH₄diffusion and ebullition. DielpCO₂,pCH₄,pN₂O, and diffusive fluxes of CO₂on the submerged floodplain were also driven by in situ respiration. The drawdown/inundation cycle of the Three Gorges Reservoir therefore changes the magnitudes of aquatic greenhouse gas fluxes on its floodplain. 

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