Size fractionation of phosphorus (dissolved, colloidal and particulate) in two tributaries to Lake Lugano

An in-situ fractionation mainly consisting of continuous flow centrifugation and cross-flow-ultrafiltration was used to separate a particulate (particle diameter > 1 mu m), a colloidal (particle diameter < 1 pm and molecular weight > 10 kD) and a dissolved fraction (molecular weight < 10 kD) of total phosphorus in river water. A similar fractionation pattern is observed in function of discharge in two tributaries to Lake Lugano, the rivers Cassarate and Vedeggio. In Cassarate river 31% of the total P is particulate, 10% colloidal and 59% dissolved at low discharge, whereas at high discharge 73% is particulate, 10% colloidal and 17% dissolved. 19% of the total P is particulate, 12% colloidal and 69% dissolved at low discharge in Vedeggio river, and at high discharge 19% of the total P is dissolved, 8% colloidal and 73% particulate. Release of ortho-phosphate (o-P) from Cassarate and Vedeggio colloids was observed at pH 10, and by the enzyme alkaline phosphatase in laboratory experiments. Calculations of a model based annual P-load into Lake Lugano showed an average total P-distribution with 60% particulate P, 10% colloidal P and 30% dissolved P in Cassarate and Vedeggio river for 1996 and 1997.