Figure 4: Increased dissolved oxygen concentrations were
associated with decreased dissolved organic carbon (a) and total iron
(Fe; b) in the hypolimnia of Falling Creek Reservoir (FCR; left) and
Beaverdam Reservoir (BVR; right) during the summer stratified period of
2019.
3.3 Multiannual hypoxia is
associated with increased sediment OC in FCR and BVR
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
S2). BVR exhibited summer hypolimnetic hypoxia throughout the duration
of the study (Figure S2).
In FCR, the amount of OC in sediment increased by 57% as DO
concentrations decreased from 2019 to 2021 (Figure 5b; Table S1, S2).
Consequently, total OC was lower in FCR than BVR in 2019, but not in
2021 (Figure 5b; Table S1, S2). However, the amount of Fe-OC per gram of
sediment did not change (Figure 5a; Table S1). As a result, the
percentage of sediment OC that was bound to Fe decreased from 2019 to
2021 in FCR (Figure 5c; Table S1, S3). None of these three sediment
characteristics differed between 2019 and 2021 in BVR (Figure 5d–f;
Table S1, S2, S3).