Factsheet: Phosphorus

What is Phosphorus?

Phosphorus is an element with the symbol P that attaches to soil particles and is naturally present in water in low concentrations. Together with nitrogen, it is an essential nutrient for plant life and is measured as either total phosphorus (TP), or dissolved reactive phosphorus (DRP).

Total Phosphorus

Total phosphorus (TP) is a measure of all types phosphorus present. It includes the phosphate that is stuck to soil (sediment) as well as dissolved reactive phosphorus (DRP) which is more readily available for plants. TP is an important measure because most phosphate enters our rivers attached to sediment via run-off.

Dissolved reactive phosphorus (DRP)

Over time the phosphate that is bound to the sediment dissolves, and becomes available for aquatic plant and algae growth. This is particularly an issue in slow flowing rivers where the phosphorus bound to sediment can gradually dissolve, feeding aquatic weeds and algae for many years. DRP concentrations are an indication of a waterbody’s ability to support algae and plant growth.

Why is too much phosphorus a problem?

When phosphorus levels increase to very high levels, the waterbody is likely to experience rapid weed growth or algal blooms which can choke aquatic life and cause long-term damage to the health of a stream, river, or lake. Much of the phosphorus in our rivers is a result of erosion and fertiliser use. Other sources include dairy factories, freezing works, sewage treatment plants, and pulp and paper plants. While New Zealand soils tend to be naturally low in phosphorus, decades of topdressing with superphosphate and erosion have resulted in cumulative impacts.

How to test for phosphorus?

Water samples are collected by local authorities and sent to laboratories for testing. The majority of councils use chemical test methods that follow the American Public Health Association (APHA) standards.  Local authorities, together with the Ministry for the Environment, are currently working towards standardizing sampling and testing methodologies for nutrients, including phosphorus.

Which unit is it given in?

All forms of phosphorus are measured in g/m3 (the same as mg/L) or parts per billion (ppb). 1 ppb = 0.001 g/m3.

Where do I find more information?

ANZECC & ARMCANZ (2000). Australian and New Zealand guidelines for fresh and marine water quality. Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand.


Biggs, BJF (2000). New Zealand periphyton guideline: Detecting, monitoring and managing enrichment of streams. Ministry for the Environment. 122 p.

Davies-Colley, R  2000. “Trigger” values for New Zealand rivers. Prepared for the Ministry for the Environment. NIWA Client Report: MfE002/22 May 2000.

Hudson, N., Ballantine, D., Gibbs, M., de Winton, M., Storey, R., Verburg, P., Hamill, K., Investigation of single indicators for water quality assessment and reporting. Prepared for Ministry for the Environment. NIWA Client Report No: HAM2011-066. 170p.

Matheson F, Quinn J, Hickey C 2012. Review of the New Zealand instream plant and nutrient guidelines and development of an extended decision making framework: Phases 1 & 2 final report.  Prepared for the Ministry of Science and Innovation Envirolink Fund.