Ferrous iron oxidation rates in the pycnocline of a permanently stratified lake

Ferrous iron was found year round at 2–4 mM in the anoxic hypolimnion of the Halls Brook Holding Area (HBHA), a small lake in eastern Massachusetts. Oxygenated epilimnion waters always had total iron concentrations of <80 nanomolar, implying nearly complete oxidation of ferrous iron as it mixed upward across the lake’s pycnocline. Assuming conductivity was a conservative parameter, and using data on the lake’s water balance, upward advection rates (0.02–0.05 m d−1) and vertical eddy diffusion coefficients (0.007–0.05 m2 d−1) were determined for the lake’s pycnocline on five dates. Using the same advection and diffusion parameters, corresponding pseudo first-order rate coefficients for ferrous iron oxidation, kox (s−1), on those dates were calculated (0.0004–0.007 s−1). The values of kox (s−1) were always too large to reflect only homogeneous solution reactions; and on at least four dates they appeared too fast to be due to heterogeneous catalysis on iron oxyhydroxides. This suggested that ferrous iron oxidation in this lake’s pycnocline was primarily due to catalysis by microorganisms, and this was supported by comparison of azide-poisoned vs. untreated batch tests. As a result of their continuous production, iron oxyhydroxide precipitates and any associated sorbates/coprecipitates are most likely continuously settling back into the lake’s deep water and bed sediments, except when episodic storm events flush these solids out of the pycnocline and downstream via the Aberjona River.