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 ≥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 l−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−2 day−1) and the overall loading at the lake inflow was 13.2 g km−2 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, but 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.