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  1. Large river systems, particularly those shared by developing nations in the tropics, exemplify the interconnected and thorny challenges of achieving sustainability with respect to food, energy, and water ( 1 ). Numerous countries in South America, Africa, and Asia have committed to hydropower as a means to supply affordable energy with net-zero emissions by 2050 ( 2 ). The placement, size, and number of dams within each river basin network have enormous consequences for not only the ability to produce electricity ( 3 ) but also how they affect people whose livelihoods depend on the local river systems ( 4 ). On page 753 of this issue, Flecker et al. ( 5 ) present a way to assess a rich set of environmental parameters for an optimization analysis to efficiently sort through an enormous number of possible combinations for dam placements and help find the combination(s) that can achieve energy production targets while minimizing environmental costs in the Amazon basin.
    Free, publicly-accessible full text available February 18, 2023
  2. Carbon dioxide (CO 2 ) supersaturation in lakes and rivers worldwide is commonly attributed to terrestrial–aquatic transfers of organic and inorganic carbon (C) and subsequent, in situ aerobic respiration. Methane (CH 4 ) production and oxidation also contribute CO 2 to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO 2 and CH 4 from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO 2 and CH 4 , aerobic respiration, and CH 4 production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO 2 in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO 2 supersaturation of 11,000 ± 9,000 μ atm could not be explained by aerobic respiration alone. 13 C depletion of dissolved CO 2 relative to other sources of organic and inorganic C, together with corresponding 13 C enrichment of CH 4 , suggested extensive CH 4 oxidation. A stable isotope-mixing model shows that the oxidation of 13 C depleted CH 4 to CO 2 contributes between 47 and 67% of dissolved CO 2 in Tonle Sap Lake.more »13 C depletion of dissolved CO 2 was correlated to independently measured rates of CH 4 production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH 4 production and oxidation occurs elsewhere, within inundated soils and other floodplain habitats. Seasonal inundation of floodplains is a common feature of tropical freshwaters, where high reported CO 2 supersaturation and atmospheric emissions may be explained in part by coupled CH 4 production and oxidation.« less
    Free, publicly-accessible full text available February 22, 2023
  3. Abstract

    Despite efforts to understand the hydrologic impact of hydropower dams, their influence on downstream river temperatures has gone unnoticed in data limited regions. Using 30 years of Landsat thermal infrared observations (1988–2018), we identified a relationship between dry season water temperature cooling trends and dam development in the 3S Basin, a major tributary of the Mekong River. Within a year of the beginning of operations of major dams in the 3S River Basin, rapid decreases in annual average dry season river temperature were observed ranging between 0.7 ° C and 2 ° C. Furthermore,in situwater temperature observations confirmed decreasing river temperature for two major dam development events. Evidence was found that the 3S outflow has been cooling the Mekong River downstream of the confluence, by as much as 0.8 ° C in recent years. Our findings are critically important for understanding how fish and aquatic ecosystems will behave in the future as more hydropower dams are built in the Mekong River Basin.

  4. Williams et al . claim that the data used in Sabo et al . were improperly scaled to account for fishing effort, thereby invalidating the analysis. Here, we reanalyze the data rescaled per Williams et al . and following the methods in Sabo et al . Our original conclusions are robust to rescaling, thereby invalidating the assertion that our original analysis is invalid.