Iron redox effects on photosensitive phosphorus release from dissolved humic materials

Factors controlling the release of P from dissolved humic material (DHM)‐iron complexes were studied in an acid bog lake. Gel‐permeation chromatography of lake water labeled with 32 PO 4 3− was used to separate high‐ and low‐molecular‐weight fractions of P compounds. Fe 3+ additions and irradiation with ultraviolet (UV) light increased the release of a labeled compound coeluting with orthophosphate from void volume fractions. Oxidation rates of Fe 2+ in whole‐lake water were much slower than photoreduction rates, which suggests that, in UV light, Fe accumulated as Fe 2+ . Fe 2+ concentrations decreased concomitant with a June increase in Chl a in the lake water. Biotic uptake of Fe 2+ may deplete the ambient pool of both Fe 2+ and Fe 3+ in summer and make photosensitive processes less important at this time.