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Depth profiles of dissolved organic and inorganic carbon and total and dissolved nitrogen and phosphorus were sampled from 2013-2023 in five drinking water reservoirs in southwestern Virginia, USA. The five drinking water reservoirs are: Beaverdam Reservoir (Vinton, Virginia), Carvins Cove Reservoir (Roanoke, Virginia), Falling Creek Reservoir (Vinton, Virginia), Gatewood Reservoir (Pulaski, Virginia), and Spring Hollow Reservoir (Salem, Virginia). Beaverdam, Carvins Cove, Falling Creek, and Spring Hollow Reservoirs are owned and operated by the Western Virginia Water Authority as primary or secondary drinking water sources for Roanoke, Virginia, and Gatewood Reservoir is a drinking water source for the town of Pulaski, Virginia. The dataset consists of depth profiles of water chemistry samples measured at the deepest site of each reservoir adjacent to the dam. Additional water chemistry samples were collected at a gauged weir on Falling Creek Reservoir's primary inflow tributary, as well as surface samples at multiple upstream and inflow sites in Falling Creek Reservoir 2014-2023 and Beaverdam Reservoir in 2019 and 2020. One upstream site at Beaverdam Reservoir was sampled at depth in 2022. Inflow sites at Carvins Cove Reservoir were sampled from 2020-2023. The water column samples were collected approximately fortnightly from March-April, weekly from May-October, and monthly from November-February at Falling Creek Reservoir and Beaverdam Reservoir, approximately fortnightly from May-August in most years at Carvins Cove Reservoir, and approximately fortnightly from 2014-2016 in Gatewood and Spring Hollow Reservoirs, though sampling frequency and duration varied among reservoirs and years. 

Surface samples and depth profiles of carbon dioxide and methane concentrations were sampled from 2015 to 2023 in two drinking water reservoirs in southwestern Virginia, USA: Beaverdam Reservoir (Vinton, Virginia) and Falling Creek Reservoir (Vinton, Virginia). Both reservoirs are owned and operated by the Western Virginia Water Authority as primary or secondary drinking water sources for Roanoke, Virginia. The dataset consists of depth profiles of dissolved greenhouse gas (carbon dioxide, methane) samples measured at the deepest site of each reservoir adjacent to the dam. Additional surface samples were collected at a gauged weir on Falling Creek Reservoir's primary inflow tributary, from a wetland adjacent to Falling Creek Reservoir, and from the reservoir outflow. At Beaverdam Reservoir, additional samples were collected at three outflow points below the dam and at the mid-reservoir outflow. Samples were collected approximately fortnightly from March-April, weekly from May-October, and monthly in November-February at Falling Creek Reservoir and Beaverdam Reservoir. In 2019, surface samples along the stream and reservoir continuum from both Falling Creek Reservoir and Beaverdam Reservoir were collected monthly during the summer stratified period (see site descriptions file for geographic coordinates of sampling sites). 

Depth profiles of dissolved organic carbon and total and dissolved nitrogen and phosphorus were sampled from 2013 to 2022 in five drinking water reservoirs in southwestern Virginia, USA. Some additional dissolved nitrogen and phosphorus samples from January to March 2023 are included in this data product. The five drinking water reservoirs are: Beaverdam Reservoir (Vinton, Virginia), Carvins Cove Reservoir (Roanoke, Virginia), Falling Creek Reservoir (Vinton, Virginia), Gatewood Reservoir (Pulaski, Virginia), and Spring Hollow Reservoir (Salem, Virginia). Beaverdam, Carvins Cove, Falling Creek, and Spring Hollow Reservoirs are owned and operated by the Western Virginia Water Authority as primary or secondary drinking water sources for Roanoke, Virginia, and Gatewood Reservoir is a drinking water source for the town of Pulaski, Virginia. The dataset consists of depth profiles of water chemistry samples measured at the deepest site of each reservoir adjacent to the dam. Additional water chemistry samples were collected at a gauged weir on Falling Creek Reservoir's primary inflow tributary, as well as surface samples at multiple upstream and inflow sites in Falling Creek Reservoir 2014-2022 and Beaverdam Reservoir in 2019 and 2020. One upstream site at BVR was sampled at depth in 2022. Inflow sites at Carvins Cove Reservoir were sampled from 2020 - 2022. The water column samples were collected approximately fortnightly from March-April, weekly from May-October, and monthly from November-February at Falling Creek Reservoir and Beaverdam Reservoir, approximately fortnightly from May-August in most years at Carvins Cove Reservoir, and approximately fortnightly from 2014-2016 in Gatewood and Spring Hollow Reservoirs, though sampling frequency and duration varied among reservoirs and years. Depth profiles of dissolved inorganic carbon were also collected from 2018-2022, but the analytical method for this analyte is still in development and these concentrations should be considered as preliminary data only. 

Surface samples and depth profiles of carbon dioxide and methane concentrations were sampled from 2015 to 2022 in two drinking water reservoirs in southwestern Virginia, USA: Beaverdam Reservoir (Vinton, Virginia) and Falling Creek Reservoir (Vinton, Virginia). Both reservoirs are owned and operated by the Western Virginia Water Authority as primary or secondary drinking water sources for Roanoke, Virginia. The dataset consists of depth profiles of dissolved greenhouse gas (carbon dioxide, methane) samples measured at the deepest site of each reservoir adjacent to the dam. Additional surface samples were collected at a gauged weir on Falling Creek Reservoir's primary inflow tributary, from a wetland adjacent to Falling Creek Reservoir, and from the reservoir outflow. At Beaverdam Reservoir, additional samples were collected at three outflow points below the dam and at the mid-reservoir outflow. Samples were collected approximately fortnightly from March-April, weekly from May-October, and monthly in November-February at Falling Creek Reservoir and Beaverdam Reservoir. In 2019, surface samples along the stream and reservoir continuum from both Falling Creek Reservoir and Beaverdam Reservoir were collected monthly during the summer stratified period (see site descriptions file for geographic coordinates of sampling sites). 

Abstract Freshwater lakes and reservoirs play a disproportionate role in the global organic carbon (OC) budget, as active sites for carbon processing and burial. Associations between OC and iron (Fe) are hypothesized to contribute substantially to the stabilization of OC in sediment, but the magnitude of freshwater Fe‐OC complexation remains unresolved. Moreover, global declines in bottom‐water oxygen concentrations have the potential to alter OC and Fe cycles in multiple ways, and the net effects of low‐oxygen (hypoxic) conditions on OC and Fe are poorly characterized. Here, we measured the pool of Fe‐bound OC (Fe‐OC) in surficial sediments from two eutrophic reservoirs, and we paired whole‐ecosystem experiments with sediment incubations to determine the effects of hypoxia on OC and Fe cycling over multiple timescales. Our experiments demonstrated that short periods (2–4 weeks) of hypoxia can increase aqueous Fe and OC concentrations while decreasing OC and Fe‐OC in surficial sediment by 30%. However, exposure to seasonal hypoxia over multiple years was associated with a 57% increase in sediment OC and no change in sediment Fe‐OC. These results suggest that the large sediment Fe‐OC pool (∼30% of sediment OC in both reservoirs) contains both oxygen‐sensitive and oxygen‐insensitive fractions, and over multiannual timescales OC respiration rates may play a more important role in determining the effect of hypoxia on sediment OC than Fe‐OC dissociation. Consequently, we anticipate that global declines in oxygen concentrations will alter OC and Fe cycling, with the direction and magnitude of effects dependent upon the duration of hypoxia. 

Abstract Oxygen availability is decreasing in many lakes and reservoirs worldwide, raising the urgency for understanding how anoxia (low oxygen) affects coupled biogeochemical cycling, which has major implications for water quality, food webs, and ecosystem functioning. Although the increasing magnitude and prevalence of anoxia has been documented in freshwaters globally, the challenges of disentangling oxygen and temperature responses have hindered assessment of the effects of anoxia on carbon, nitrogen, and phosphorus concentrations, stoichiometry (chemical ratios), and retention in freshwaters. The consequences of anoxia are likely severe and may be irreversible, necessitating ecosystem‐scale experimental investigation of decreasing freshwater oxygen availability. To address this gap, we devised and conducted REDOX (the Reservoir Ecosystem Dynamic Oxygenation eXperiment), an unprecedented, 7‐year experiment in which we manipulated and modeled bottom‐water (hypolimnetic) oxygen availability at the whole‐ecosystem scale in a eutrophic reservoir. Seven years of data reveal that anoxia significantly increased hypolimnetic carbon, nitrogen, and phosphorus concentrations and altered elemental stoichiometry by factors of 2–5× relative to oxic periods. Importantly, prolonged summer anoxia increased nitrogen export from the reservoir by six‐fold and changed the reservoir from a net sink to a net source of phosphorus and organic carbon downstream. While low oxygen in freshwaters is thought of as a response to land use and climate change, results from REDOX demonstrate that low oxygen can also be adriverof major changes to freshwater biogeochemical cycling, which may serve as an intensifying feedback that increases anoxia in downstream waterbodies. Consequently, as climate and land use change continue to increase the prevalence of anoxia in lakes and reservoirs globally, it is likely that anoxia will have major effects on freshwater carbon, nitrogen, and phosphorus budgets as well as water quality and ecosystem functioning.