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Media type:
E-Article
Title:
Light-Induced Redox Cycling of Iron in Circumneutral Lakes
Contributor:
Emmenegger, Lukas;
Schonenberger, Rene;
Sigg, Laura;
Sulzberger, Barbara
imprint:
American Society of Limnology and Oceanography, 2001
Published in:Limnology and Oceanography
Language:
English
ISSN:
0024-3590
Origination:
Footnote:
Description:
<p>The light-induced redox cycling of Fe<sup>II</sup>/Fe<sup>III</sup>was studied both in laboratory experiments and in the field in two circumneutral Swiss lakes: Greifensee, a eutrophic, natural water body, and Melchsee, an oligotrophic, artificial mountain lake. To determine Fe<sup>II</sup>at the nanomolar level, an automated flow-injection analysis system was used. Irradiation by simulated sunlight leads to pH dependent (pH 6.9-9.1) steady-state Fe<sup>II</sup>concentrations which are similar in samples from both lakes. However, the kinetics of Fe<sup>III</sup>reduction and of apparent Fe<sup>II</sup>oxidation are considerably faster in Melchsee. On the basis of experimental results and on modeling that uses literature values of known chemical transformation processes, we suggest that superoxide may be a key parameter for light-induced iron redox cycling in these lakes. Field measurements of [Fe<sup>II</sup>] in Greifensee and Melchsee show a pronounced day/ night cycle, with Fe<sup>II</sup>concentrations of ∼0.1-0.2 nM at night and up to 0.9 nM near the surface during the day (pH 8.0-8.5). Depth profiles of [Fe<sup>II</sup>] have two maxima: one at the surface and the second one at a depth of 5-10 m. Empirical rates and measured physical parameters were included in a model to simulate [Fe<sup>II</sup>] as a function of time and depth. The model results indicate that Fe<sup>II</sup>at the surface of both lakes is produced by light-induced processes, whereas the deeper Fe<sup>II</sup>maxima at depths with maximal chlorophyll a concentrations are probably due to a combination of biologically and photochemically induced processes.</p>