Figure 2: Iron-bound organic carbon (Fe-OC; a), total sediment
organic carbon (b), and Fe-OC as a percentage of sediment OC (c) all
differed significantly between surificial sediment and settling
particulate matter. Asterisks indicate statistical significance of the
difference between surficial sediment and sediment traps: *** indicates
p < 0.001 (Welch’s ANOVA). Note that only 2021 data are
presented because settling particulate material was not collected in
2019.
3.2 Short-term hypoxia decreases
sediment OC and Fe-OC on a whole-ecosystem scale
Intermittent activation of the HOx in FCR in 2019 was associated with
substantial changes in sediment characteristics. The amount of Fe-OC per
g sediment was 30% lower during hypoxic (µ=394±173, n=9) compared to
oxic (µ=560±70, n=7) periods in 2019 (t11=-2.6, p=0.02;
Figure 3b). Likewise, the total amount of OC (as a percentage of
sediment mass) decreased by 30% during hypoxic (µ=6.0±1.55, n=11)
compared to oxic periods (µ=8.5±0.8, n=7; t15=-4.6,
p<0.001; Figure 3b). Fe-OC as a percentage of total sediment
OC did not significantly change with variation in oxygen during these
experiments (oxic: µ=36.7±3.8, n=7; hypoxic: µ=35.7±7.9, n=9;
t12=-0.3, p=0.747; Figure 3c).
In the hypolimnion of FCR, total Fe concentrations tended to increase as
oxygen decreased and decrease as oxygen increased (Figure 4b).
Consequently, Fe concentrations were generally lower in FCR compared to
the unoxygenated reference reservoir (BVR) in 2019. Trends in DOC were
more variable, though DOC concentrations were typically highest when
oxygen concentrations were low in FCR (Figure 4).