Light-induced redox cycling of iron in circumneutral lakes

The light-induced redox cycling of Fe-II/Fe-III 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-II at the nanomolar level, an automated Bow-injection analysis system was used. irradiation by simulated sunlight leads to pH dependent (pH 6.9-9.1) steady-state Fe-II concentrations which are similar in samples from both lakes. However, the kinetics of Fe-III reduction and of apparent Fe-II 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-II] in Greifensee and Melchsee show a pronounced day/night cycle, with Fe-II concentrations of similar to0.1-0.2 nM at night and up to 0.9 nM near the surface during the day (pH 8.0-8.5). Depth profiles of [Fe-II] 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-II] as a function of time and depth. The model results indicate that Fe-II at the surface of both lakes is produced by light-induced processes, whereas the deeper Fe-II maxima at depths with maximal chlorophyll a concentrations are probably due to a combination of biologically and photochemically induced processes.