Light-induced redox cycling of iron in circumneutral lakes

The light-induced redox cycling of Fe-II/Fe-III was studied both in laborat ory experiments and in the field in two circumneutral Swiss lakes: Greifens ee, a eutrophic, natural water body, and Melchsee, an oligotrophic, artific ial mountain lake. To determine Fe-II at the nanomolar level, an automated Bow-injection analysis system was used. irradiation by simulated sunlight l eads to pH dependent (pH 6.9-9.1) steady-state Fe-II concentrations which a re similar in samples from both lakes. However, the kinetics of Fe-III redu ction and of apparent Fe-II oxidation are considerably faster in Melchsee. On the basis of experimental results and on modeling that uses literature v alues of known chemical transformation processes, we suggest that superoxid e may be a key parameter for light-induced iron redox cycling in these lake s. Field measurements of [Fe-II] in Greifensee and Melchsee show a pronounc ed day/night cycle, with Fe-II concentrations of similar to0.1-0.2 nM at ni ght and up to 0.9 nM near the surface during the day (pH 8.0-8.5). Depth pr ofiles 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 i ncluded 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.