Figure 5: Increased dissolved oxygen concentrations (black lines) were associated with decreased dissolved organic carbon (brown lines; a) and total iron (Fe; slate blue lines; b) in the hypolimnia of Falling Creek Reservoir (FCR; left) and Beaverdam Reservoir (BVR; right) during the summer stratified period of 2019. All data presented are from the deepest sampling depth in each reservoir (9 m in FCR and 11 m in BVR).
3.3 Multiannual hypoxia was associated with increased sediment OC
Activation of the oxygenation system increased summer hypolimnetic DO concentrations in FCR from 2014 through 2019, and lower oxygen addition rates allowed for primarily hypoxic conditions in 2020 and 2021 (Figure S1). BVR exhibited summer hypolimnetic hypoxia throughout the duration of the study (Figure S1).
In FCR, the amount of OC in surficial sediment increased by 57% as DO concentrations decreased from 2019 to 2021 (Figure 6b; Table S1, S2). Consequently, total OC was lower in FCR than BVR in 2019, but not in 2021 (Figure 6b; Table S1, S2). However, the amount of Fe-OC per gram of sediment did not change (Figure 6a; Table S1). As a result, the percentage of sediment OC that was bound to Fe decreased from 2019 to 2021 in FCR (Figure 6c; Table S1, S3). None of these three sediment characteristics differed between 2019 and 2021 in BVR (Figure 6d–f; Table S1, S2, S3).