Figure 1: Conceptual diagram describing the hypothesized effects of changing oxygen conditions on coupled iron (Fe) and organic carbon (OC) interactions in a lake or reservoir. Under oxic conditions (top), complexation of Fe and OC (both through coprecipitation and adsorption) leads to increased concentrations of Fe-OC in sediments (increased Fe-OC protection), though oxic conditions may also lead to increased OC respiration rates. Under hypoxic conditions, reductive dissolution of Fe(III) in Fe-OC complexes increases dissolved concentrations of Fe(II) and OC in the water column while decreasing the amount of Fe-OC in sediment (decreasing Fe-OC protection), though hypoxia may also decrease OC respiration rates. The net effect of these processes on OC sequestration remains unknown, motivating this study. This figure is a simplification of complex interactions happening on a whole-ecosystem scale, and focuses on hypothesized dominant processes operating on the timescale of days to years.
2. Methods
2.1 Study Sites: Falling Creek and Beaverdam Reservoirs
Falling Creek Reservoir (FCR; 37.30ºN, 79.84ºW) and Beaverdam Reservoir (BVR; 37.31ºN, 79.81ºW) are small (FCR: 0.12 km2, 9.3 m deep; BVR: 0.39 km2, 11 m deep), eutrophic drinking water reservoirs located in southwestern Virginia, USA (Hounshell et al., 2021; Figure 2). Both reservoirs are located in deciduous forested catchments and both are dimictic, with summer stratified periods that typically last from May to October. BVR is located 3 km upstream of FCR and serves as the primary inflow source for FCR. Fe levels are high in surface water and groundwater from this region as a result of weathering and erosion of Fe-rich metamorphic rocks (53,466 mg/kg dry weight in sediment; Krueger et al., 2020; Chapman et al., 2013; Woodward, 1932). The bedrock underlying both reservoirs is layered pyroxene granulite (Virginia Division of Mineral Resources, 2003), and the primary soil series in the reservoir catchments is Edneytown Loam (Soil Survey Staff). The Köppen-Geiger climate designation of this region is Cfa: humid subtropical climate (Kottek et al. 2006). Both reservoirs have been owned and operated for drinking water provision by the Western Virginia Water Authority (WVWA) since their construction (FCR: 1898, BVR: 1872; Gerling et al., 2016; Hamre et al., 2018).
A suite of variables are routinely sampled in FCR and BVR as part of a long-term monitoring program; all data analyzed in this manuscript are available in the Environmental Data Initiative (EDI) repository with detailed metadata (Carey, Lewis, McClure, et al., 2022; Carey, Wander, Howard, et al., 2022; Carey, Wander, McClure, et al., 2022; Lewis et al., 2022; Lewis, Schreiber, et al., 2022; Schreiber et al., 2022).