Effects of hypoxia on coupled carbon and iron cycling differ between
weekly and multiannual timescales in two freshwater reservoirs
Abstract
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.