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.
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).