Figure 5: Sediment organic carbon metrics differed
significantly in association with oxygen on a multiannual scale. Metrics
assessed include iron-bound organic carbon (Fe-OC) (a), total sediment
organic carbon (OC; b), and Fe-OC as a percentage of OC (c) in Falling
Creek Reservoir (FCR) and Beaverdam Reservoir (BVR). Blue color
indicates the reservoir-year with the highest mean oxygen (2019 in FCR)
and letters delineate groups that are significantly different (p
< 0.05; Table S1, S2, S3).
3.4 Experimental microcosm
incubations reveal rapid effects of hypoxia on Fe and OC
Experimental microcosm incubations successfully established four
distinct oxygen regimes. DO concentrations increased rapidly when
hypoxic microcosms were unsealed and decreased rapidly when microcosms
were sealed (Figure 6). At the transition from hypoxic-to-oxic
conditions, DO concentrations increased to approximately the same level
as the continuous oxygen treatment (~7 mg/L) within one
day. At the transition from oxic-to-hypoxic conditions, DO
concentrations decreased below 1 mg/L within one day and declined to 0
mg/L by the end of the experiment.
Changes in oxygen conditions were associated with clear but asynchronous
changes in aqueous OC and Fe. As microcosms switched from
oxic-to-hypoxic conditions, TOC, DOC and total Fe decreased near
synchronously, while dissolved Fe decreased below detection within one
day of oxygen exposure. At the transition from hypoxic-to-oxic
conditions, DOC and TOC rapidly increased to the same level as
microcosms that had experienced continuous hypoxia (~10
mg/L; Figure 6). However, concentrations of both dissolved and total Fe
only began to increase after three weeks of hypoxia (Figure 6). Measured
DOC and TOC were strongly and linearly correlated, with DOC representing
a mean of 96±14% of TOC (Figure S4); thus, we focus our discussion on
DOC hereafter, but the same trends apply to TOC.
At the end of the experiment, sediment OC differed significantly among
treatments (one-way ANOVA: F3,20=9.09,
p<0.001). Sediment OC was significantly higher in microcosms
that started under oxic conditions (oxic: µ=4.6±0.3, oxic-to-hypoxic:
µ=4.5±0.3) than microcosms that started under hypoxic conditions
(hypoxic: µ=4.0±0.0, hypoxic-to-oxic: µ=4.1±0.2; Figure 7). Fe-OC did
not differ significantly between treatments as a proportion of sediment
mass (F3,20=0.51, p=0.683) or as a proportion of
sediment OC (F3,20=2.40, p=0.098).
Speciation calculations (Table S5) based upon ICP-MS results (Figure S5)
suggest that oxygen conditions had primary control over Fe speciation,
with a lesser impact on Fe-OC. The experiments that were maintained
under hypoxic conditions had dominant Fe species of Fe(II),
FeHCO3+, FeCO3, and
FeSO4 (all of these species contained Fe in its reduced
state, Fe(II)). For all of the microcosms that were exposed to oxygen at
any time (hypoxic-to-oxic, oxic-to-hypoxic, oxic), the dominant Fe
species were Fe(OH)3,
Fe(OH)2+,
Fe(OH)4-,
FeOH2+ and FeHumate+(all of these species contained Fe in the oxidized state, Fe(III)). pH
remained circumneutral across all treatments (Figure S6). These results
indicate that 1) exposure to oxic conditions at any time in the
experiment shifted the dominant oxidation state to Fe(III); 2) under
oxic conditions, and to a lesser extent, hypoxic conditions, Fe
complexed with DOC.