Freshwater lakes and reservoirs play a disproportionate role in the global carbon budget, sequestering more organic carbon (OC) than ocean sediments each year. However, it remains unknown how global declines in bottom-water oxygen concentrations may impact OC sequestration in freshwater sediments. In particular, associations between OC and iron (Fe) are hypothesized to play a critical role in stabilizing OC in sediment, and these complexes can be sensitive to changes in oxygen. Under low-oxygen (hypoxic) conditions, Fe-bound OC (Fe-OC) complexes may dissociate, decreasing OC sequestration. However, rates of OC respiration are also lower under hypoxic conditions, which could increase OC sequestration. To determine the net effects of hypoxia on OC and Fe cycling over multiple timescales, we paired whole-ecosystem experiments with sediment incubations in two eutrophic reservoirs. Our experiments demonstrated that short (2–4 week) periods of hypoxia can increase Fe and OC concentrations in the water column while decreasing OC and Fe-OC in sediment by 30%. However, exposure to seasonal hypoxia over multiple years was associated with a 57% increase in sediment OC and no change in Fe-OC. These results indicate that the large sediment Fe-OC pool (~30% of sediment OC) contains both oxygen-sensitive and oxygen-insensitive fractions, and over multiannual timescales, OC respiration rates play a greater role than Fe-OC protection in determining the effect of hypoxia on sediment OC content. Consequently, we anticipate that global declines in oxygen concentrations will alter OC and Fe cycling, with the direction and magnitude of effects depending upon the duration of hypoxia.

The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn cycling dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and PLSR modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two distinct transitional periods: reservoir turnover (Fall 2020) and initiation of the HOx system (Summer 2021). Our sensor system was able to successfully predict mean Fe and Mn concentrations as well as capture sub-weekly variability, ground-truthed by traditional grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir occurred, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 hours after full water column mixing. During the initiation of hypolimnetic oxygenation in Summer 2021, we observed that Fe and Mn were similarly affected by physical mixing in the hypolimnion, but displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are highly sensitive to shifting DO and stratification and that their dynamics can substantially change on hourly to daily time scales in response to these transitions.