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

This study illustrates an integrated watershed-based approach for assessment of phosphorus (P) in the Lake Hayes catchment in the South Island of New Zealand. Spatial and temporal variability of total (TP) and dissolved reactive P (DRP) concentrations and loadings were evaluated at key locations including inflow to, within, and outflow from the lake, and in tributaries. Particulate P was the dominant form, and TP was highly correlated with suspended solids. Equivalence tests were used to evaluate differences in TP concentrations among locations and showed differences greater than or equal to 20% between only two streams. Differences between other locations were not as large but some were still significant. Concentrations were highest at the lake inflow (mean = 0.065 and maximum = 0.614 mg 1(-1)), and values at the lake surface were low but increased with depth. The lower half of the catchment had the greatest specific TP loading (25.2 g km(-1) day(-1)) and the overall lending at the lake inflow was 13.2 g km(-1) day(-1). Temporal variability, particularly during a storm, was evaluated in detail at the lake inflow. Seasonality in concentrations for the group of stations was not evident, hut values at Fish Trap increased considerably during the storm (81 540 g day(-1)). Baseflow, snowmelt, and one storm accounted for 47, 30 and 23%, respectively, of the total annual loading. Evaluation of data attributes was performed to assist with interpretation of results and showed that the data were very right-skewed and there was considerable uncertainty. Evaluation of groundwater quality data provided information on potential P sources to and losses from surface water and suggested that P enters groundwater in the upper catchment and is discharged to surface water in the lower reaches. Although concentrations exceeded aquatic ecosystem guidelines in several locations, assessment of aquatic biota suggested that they are not limiting stream benthic communities or causing algal blooms in recent years. The methodology provides a framework that integrates components of other studies of P processes and export, and can be used for NPS pollution assessment, targeting and decision making in agricultural catchments. (C) 2000 Elsevier Science B.V. All rights reserved.