Integrated assessment of phosphorus in the Lake Hayes catchment, South Island, New Zealand

An integrated, watershed-based methodology for assessing agricultural nonpoint source phosphorus (P) pollution is presented and applied to a New Zealand watershed. Spatial and temporal variability in total P (TP) and dissolved reactive P concentrations and loadings were measured at several key sites in the Lake Hayes catchment. Particulate P dominated. TP was strongly correlated with suspended solids. The highest concentrations occurred at the lake inflow. Lake surface values were low but increased with depth. The highest and lowest TP concentrations differed by about 20%. Seasonal variability in loadings was not strong but storms accounted for considerable temporal variability. Analysis of groundwater quality data suggests that P enters groundwater in the upper catchment and is discharged to surface water in the lower reaches. P concentrations exceeded aquatic ecosystem guidelines at several locations, but these levels do not seem to be limiting stream benthic communities or causing algal blooms.