Figure 3: 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 surficial 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 decreased 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 4b). 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 4b). 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 4c).
In the hypolimnion of FCR, total Fe concentrations tended to increase as oxygen decreased and decrease as oxygen increased (Figure 5b). 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 5).