Current state vs freshwater objectives: Phosphorus

Phosphorus is a nutrient that plants need to grow. Added to rivers, lakes and estuaries in excess, it feeds algae and weed growth that can choke aquatic life and cause long-term damage to the health of a water body.

The dataset - what is it?

View details on the minimum standard for phosphorus in water bodies, set by government policy, and the current degree of compliance. The National Policy Statement for Freshwater Management 2020 sets a national bottom line for total phosphorus in lakes of 50 mg/m³ (annual median), and bands for dissolved reactive phosphorus in rivers with no national bottom line. The 2023 national study behind this dataset assessed rivers against total phosphorus criteria for periphyton (algae on the river bed) and lakes against the lake bottom line. You can view the total phosphorus load (TP) and where it exceeds the maximum allowable load (MAL) on a map of New Zealand.

Why it's important

Phosphorus binds to soil particles, so most of it reaches water attached to sediment in run-off from erosion, along with dung, fertiliser and effluent washed off paddocks, tracks and other critical source areas. Some regions also carry phosphorus released from their rock and soil.

The 2023 study estimates that phosphorus loads need to fall in catchments covering 11% of New Zealand’s land area, and by 6% of the current load nationally. The reductions required are largest in Otago (13%), Manawatū-Whanganui (12%) and Southland (12%). Where a catchment needs a reduction of more than 30%, some land use change is likely. Elsewhere, land management can bring phosphorus down: soil testing to keep Olsen P in the optimum range, no phosphate fertiliser when rain is forecast within 14 days, riparian planting, keeping stock out of waterways, managing critical source areas, and retiring erosion-prone land. Research indicates farmers can halve phosphorus loss for less than 2% of farm earnings.