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SUMMARY. 1. Sephadex gel filtration of filtered water from small, Finnish forest lakes demonstrated abiotic movement of 33P from added PO4 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 13P had nominal molecular weights of > 100,000 and 10,000‐20,000. 2. An equilibrium existed between free PO4 and the two fractions. However, one fraction, at least, appeared to exist in two phases, with one phase in rapid equilibrium with free PO4 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 PO4, 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 33PO4. 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 PO4 under the experimental conditions employed. 5. Possible implications for biological phosphorus demand of such sequestration of free PO4 by humic materials in combination with iron are discussed.

1. The effect of ionic strength on the configuration of aquatic humic substances was studied by gel filtration and dialysis of water from small. Finnish forest lakes of varying colour.2. Sephadex gel filtration of water from the most humic lake gave similar elution profiles of UV absorbance and dissolved organic carbon (DOC). Gel filtration of unconcentrated samples from all three lakes under natural conditions of ionic strength (/== 1.7x U)"-') and pH (5.5-6.0) gave similar fractionation of humic substances, despite their widely differing colour (3{W5()mg Pt 1 ') and DOC (5-25mg C P').3. Increasing the ionic strength by two orders of magnitude caused considerable retardation on the Sephadex columns of the humic substances, suggesting a decrease in their molecular size and/or shape.4. Dialysis experiments strongly indicated thai ionic strength-induced changes in the configuration of the aquatic humic substances are indeed real. Hence it is probable that the elution behaviour of aquatic humic substances on Sephadex gels has previously been wrongly attributed to ionic strength-dependent interactions between the gel and the humic substances.

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