Figure 7: 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
Our results suggest that oxygen affects coupled OC and Fe cycling
differently over short-term (several weeks) compared to long-term
(multiannual) timescales (Figure 8). Short periods of hypoxia decreased
total OC and Fe-OC in 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. Notably, Fe-OC comprised
nearly one-third of surficial sediment OC in both FCR and
BVR—regardless of oxygen status—which is substantially 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).