Phosphorus transformations in the epilimnion of humic lakes: abiotic interactions between dissolved humic materials and phosphate

Abstract

SUMMARY. 1. Sephadex gel filtration of filtered water from small, Finnish forest lakes demonstrated abiotic movement of 33 P from added PO 4 to two higher molecular weight fractions. This movement was most pronounced in waters with high humic content which also had high iron content. The two fractions which took up 13 P had nominal molecular weights of > 100,000 and 10,000‐20,000. 2. An equilibrium existed between free PO 4 and the two fractions. However, one fraction, at least, appeared to exist in two phases, with one phase in rapid equilibrium with free PO 4 but the other in only slow equilibrium. 3. Additions of ferric iron up to 1 mg Fe l −1 to the filtered lake water stimulated movement from free PO 4 , provided high concentrations of humic materials were present. In the absence of humic materials even 0.1 mg Fe 1 −1 would precipitate all added 33 PO 4 . 4. The high molecular weight P was only partially reactive with standard molybdate reagents. Exposure of the high molecular weight P to sunlight caused a small release of PO 4 under the experimental conditions employed. 5. Possible implications for biological phosphorus demand of such sequestration of free PO 4 by humic materials in combination with iron are discussed.