Photoreduction of iron in the epilimnion of acidic lakes

Two acid soft-water lakes in the east of Bclgillm contain appreciable amounts of soluble ferrous iron in the epilimnetic zone which change diurnally and annually. A considerable drop in rcdox potential in the same zone confirms the existence of an active reduction of ferric iron, with a concomitant solubilization of particulate iron and of deposited iron supplied from the sediments in contact with epilimnctic water when the latter is well oxygenated. The reduction nrocess is cssentiallv nhotochcmical, promoted by optimal pI1 conditions and by the prese&c of organic mattcr’(humic acids). Early limnological studies provided much information about seasonal and vertical variations of iron in lakes and about the proportion of ferrous to total iron (e.g. Mortimer 1941, 1942; Hutchinson 1957; Ruttner 1973), but the reliability of the available analytical techniques has been criticized (Stumm and Lee 1960)