The redox chemistry of Fe was investigated in Lake Kinneret (Sea of Galilee), a mesotrophic, monomictic lake in the central part of the Jordan Rift Valley. The concentrations of Fe(II) and Fe(tot) in the epilimnion and in the hypolimnion were measured, and the relationships between Fe(II) and other parameters (e.g., light, pH) were investigated. In addition, laboratory experiments were conducted where filtered (biota-free) Lake Kinneret waters, sterile unfiltered Lake Kinneret waters, and distilled waters were spiked with various concentrations of Fe(III) and Fe(II). The concentrations of Fe(III) and Fe(II) were measured as a function of time in water samples under a variety of pH, O2, and radiation conditions. Iron(II) concentrations in the epilimnion were below detection limit (0.04 μM) during nighttime, whereas in daytime Fe(II) concentrations were always above the detection limit and changed significantly around the year (0.05-0.15 μM). Fe(II)/Fe(tot) ratios measured in the lake (3-99%) are higher than the expected values for a high pH (pH ∼ 8), low ionic strength (∼10 mM) aquatic system. In addition to photo-induced reduction of Fe(III), there is a strong evidence that Fe(II) is stabilized in the O2-saturated lake water, as Fe(II) concentrations can be detected at a depth of 10 m, where less than 10% of the light penetrates. The results of the oxidation and photo-reduction experiments suggest that the observed rates of Fe redox reactions in sterile lake water are consistent with known chemical (abiotic) rates, but that these rates cannot account for the observed Fe(II) concentrations in the epilimnion waters. Therefore, we propose that the photo-induced redox cycle of Fe in the epilimnion of Lake Kinneret is largely controlled by biological activity and that abiotic photo-reduction of Fe accounts for only a small fraction of the observed Fe(II) in the epilimnion.
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The authors wish to thank A. Sukenik of The Institute of Limnology and Oceanography of Israel for his comments and in-depth discussions and to B. Steinman of the Kinneret Limnological Laboratory for providing estimates of vertical mixing rates in the lake. The authors also wish to thank the Admiral Ben-Nun Foundation for partial financial support for the research.