UV‐sensitive complex phosphorus: Association with dissolved humic material and iron in a bog lake1

The concentration of UV‐sensitive complex phosphorus compounds in water from an acid bog lake was linearly related to the concentration of dissolved high molecular weight humic material (DHM) both seasonally and diurnally. The first‐order rate of photoreduction (F 3+ to Fe 2+ ) of DHM ·· iron equaled the rate of release of orthophosphate (SRP) from these compounds. The rate of photoreduction of DHM ·· iron was the same as the rate of photoreduction of ferric citrate under similar conditions. Reduced DHM ·· iron was auto‐oxidizable (Fe 2+ to Fe 3+ ); ferrous citrate was not. These data support the view that orthophosphate sorbed to ferric iron ·· DHM complexes may be released by a mechanism involving UV‐induced photoreduction of ferric iron to the ferrous state.