Effects of Hypoxia on Coupled Carbon and Iron Cycling Differ Between Weekly and Multiannual Timescales in Two Freshwater Reservoirs

Freshwater lakes and reservoirs play a disproportionate role in the global organic carbon (OC) budget, as active sites for carbon processing and burial. Associations between OC and iron (Fe) are hypothesized to contribute substantially to the stabilization of OC in sediment, but the magnitude of freshwater Fe-OC complexation remains unresolved. Moreover, global declines in bottom-water oxygen concentrations have the potential to alter OC and Fe cycles in multiple ways, and the net effects of low-oxygen (hypoxic) conditions on OC and Fe are poorly characterized. Here, we measured the pool of Fe-bound OC (Fe-OC) in surficial sediments from two eutrophic reservoirs, and we paired whole-ecosystem experiments with sediment incubations to determine the effects of hypoxia on OC and Fe cycling over multiple timescales. Our experiments demonstrated that short periods (2-4 weeks) of hypoxia can increase aqueous Fe and OC concentrations while decreasing OC and Fe-OC in surficial sediment by 30%. However, exposure to seasonal hypoxia over multiple years was associated with a 57% increase in sediment OC and no change in sediment Fe-OC. These results suggest that the large sediment Fe-OC pool (similar to 30% of sediment OC in both reservoirs) contains both oxygen-sensitive and oxygen-insensitive fractions, and over multiannual timescales OC respiration rates may play a more important role in determining the effect of hypoxia on sediment OC than Fe-OC dissociation. Consequently, we anticipate that global declines in oxygen concentrations will alter OC and Fe cycling, with the direction and magnitude of effects dependent upon the duration of hypoxia. Plain Language Summary Freshwater lakes and reservoirs (hereafter: lakes) play a remarkably important role in the global carbon cycle, as important sites for both carbon sequestration and greenhouse gas emissions. The extent to which lakes sequester carbon versus release greenhouse gases depends upon many factors, including associations between carbon and iron that can help to preserve carbon in sediment. However, global declines in freshwater oxygen concentrations have the potential to affect these chemical complexes. Here, we added oxygen to the bottom waters of a lake to test how changes in oxygen concentration affect carbon and iron cycling. We found that over short timescales (weeks), low oxygen conditions decreased the amount of carbon in sediment by breaking apart associations between iron and carbon that help retain carbon in sediment. However, over long timescales (years), low oxygen conditions appeared to increase carbon burial by decreasing the rate at which carbon inputs were decomposed. These results suggest that declining oxygen concentrations in lakes around the world may have important effects on global carbon cycling, with the direction and magnitude of the impact depending on the duration of low oxygen conditions.