Phosphorus availability in Lake Memphremagog and its tributaries1

The percentage of total phosphorus in samples from Lake Memphremagog and five of its tributaries which was biologically available was measured on three dates, with criteria the distribution of radioactive P among exchanging fractions and the enzymatic release of soluble P from particulate P when stored with chloroform. In lake water, available P formed 83% of the total; in rivers much less (18–57%) was available. Large molecular weight phosphorus (>5,000 mol wt), separated by gel filtration, was usually the most refractory fraction, but substantial amounts of unavailable P were also associated with particles and small molecular weight P (<400 mol wt). Soluble P of intermediate molecular weight formed too small a fraction for determination of its availability. Variations in the percentage of available P in natural waters could substantially alter the biological effect of a given phosphorus load.