The use of an ultrafiltration technique for measurement of orthophosphate in shallow wetlands

The overestimation of orthophosphate by filterable reactive phosphorus FRP. measurement techniques has long been accepted. The aim of this study was to quantify that overestimation in 17 wetlands over time. Specifically an ultrafiltration technique was used prior to the application of the molybdenum blue phosphorus detection method to quantify orthophosphate concentrations. Samples were collected over a 6-month period and analysed for total filterable -0.50 mm. phosphorus TFP., filterable reactive phosphorus FRP-0.50 mm. and ultrafiltered -103 Da. reactive phosphorus PO4.. FRP correlated well with PO4, however, FRP overestimated PO4 particularly with increasing colloidal phosphorus concentration. The ratio of DOC to TFP C:P. influenced the fate of PO4 in the water, implying that DOC was forming complexes with phosphorus. The PO4 concentrations decreased with increasing C:P ratios in some of the wetlands over the 6-month monitoring period.