Figure 8: Experimental oxygen microcosm treatments altered some, but not all, sediment properties. Metrics assessed include moles of iron-bound organic carbon (Fe-OC) per unit sediment mass (a), total sediment organic carbon (b), and Fe-OC as a percentage of sediment OC (c). Letters delineate treatments that are significantly different (p < 0.05): no treatments were significantly different for Fe-OC metrics (a, c). Days 20 and 23 were chosen for statistical comparisons as the last days in the experiment when data were available from all treatments.
4. Discussion
Results from our whole-ecosystem manipulations suggest that oxygen affects coupled OC and Fe cycling differently over short-term (weekly) compared to long-term (multiannual) timescales (Figure 9). Short periods of hypoxia decreased total OC and Fe-OC in surficial sediment and increased concentrations of DOC and Fe in overlying water, indicating that a portion of the sediment Fe-OC pool is sensitive to changes in oxygen. However, over longer timescales, low oxygen conditions in FCR from 2019–2021 were associated with a 57% increase in sediment OC, indicating that the effects of hypoxia on Fe-OC (i.e., dissociation of Fe-OC complexes) may be outweighed by decreases in respiration rates under hypoxic conditions. Microcosm incubations composed of slightly deeper sediment layers showed no significant change in sediment Fe-OC in response to hypoxia, suggesting that buried Fe-OC may be resistant to the effects of hypoxia. Notably, Fe-OC comprised nearly one-third of surficial sediment OC in both FCR and BVR—regardless of oxygen status—which is more than previously reported for freshwater lakes (Peter & Sobek, 2018). Below, we discuss short-term (section 4.1) and multiannual (section 4.2) results in the context of previous work, analyze net processing rates across the sediment-water interface (section 4.3), and discuss why Fe-OC levels may be higher in these reservoirs than other freshwater systems (section 4.4).