Varied sediment archive of Fe and Mn contents under changing reservoir mixing patterns, oxygenation regimes, and runoff inputs

Water hypoxia intensification in lakes and reservoirs has become a global problem under global warming. The Mn/Fe ratio is frequently employed to reconstruct historical redox conditions, but interpretation of this ratio may be problematic when the accumulation of Fe and Mn is governed by factors in addition to redox processes. We tested the Fe and Mn contents and bacterial diversity of a 250 cm sediment column in a monomictic reservoir. The deposition time frame and sedimentary rate (approximately 60 cm/y) were determined by integrating the Fe and Mn contents with the hydrological time series, providing a sufficient archive to reconstruct the intra-year variation in water qualities. The inapplicability of the Mn/Fe ratio observed because redox processes, mixing patterns, and biological activity jointly affected the net accumulation of Fe and Mn in the sediment. The causes of the vertical variations in the sediment Fe and Mn contents are discussed, considering the runoff sediment input, hydrodynamic and thermodynamic characteristics of this reservoir, and bacterial distributions in the sediment column. High-speed deposition intensively occurred in summer and autumn; thus, during this period, fluvial sediment was the main source of Fe and Mn. Another source was biochemical sedimentation due to mixing and oxygenation in this reservoir, which played a larger role in spring and winter. Our study showed that the easily accessible current hydrological data of the reservoir provide a reference time frame for identifying historical environmental events and that the use of the Fe/Mn ratio alone is inconclusive for interpreting the historic oxygenation regimes of reservoirs. Future applications of this method should consider the individual reservoir characteristics that impact the mobility and net accumulation of Fe and Mn in the sediment.

Automated summary

Water hypoxia in lakes and reservoirs is increasing globally due to global warming. The interpretation of the Mn/Fe ratio for reconstructing historical redox conditions may be problematic because factors other than redox processes can affect the accumulation of Fe and Mn. In this study, the Fe and Mn contents and bacterial diversity in the sediment of a monomictic reservoir were examined to determine the deposition time frame and sedimentary rate. It was found that the Mn/Fe ratio alone is not sufficient for interpreting the historic oxygenation regimes of reservoirs, and factors such as runoff sediment input, hydrodynamic characteristics, and bacterial distributions should be considered.