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.  There are currently no guidelines or limits set for acceptable concentrations of DRP in groundwater, although some Regional Councils have set surface water quality limits for DRP.

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.  Where groundwater supports surface water flows, the concentration of DRP in groundwater can contribute to the growth of algae in streams.

Where does phosphorus come from?

Phosphorus can occur naturally in rocks and minerals, and can be a common component in soils and sediments. Weathering of rocks and minerals releases phosphorus in bio-available forms as DRP, suitable for uptake by plants. DRP can occur naturally in groundwater depending on the aquifer geology and groundwater conditions.

Fertilisers can be applied to soils to improve phosphorus availability, enabling agricultural and horticultural intensification and improve pasture production. Phosphorus binds strongly to soil particles, but once the capacity of the soil to store phosphorus is exceeded, it will leach downward through the soil profile into groundwater. Alternatively, if the soil suffers erosion, it will move with surface runoff to rivers. Much of the phosphorus in our rivers is a result of erosion and fertiliser use.

Typically natural DRP concentrations in groundwater are low (<0.1 mg/L), indicating that higher levels of DRP could be of anthropogenic origin. However, in some areas of New Zealand concentrations of DRP in groundwater have a natural source component, due to the chemical make-up of the aquifer geology, the redox environment and age of the groundwater.

Phosphorus, in both soluble and complex organic forms, is a key component of domestic wastewater and animal waste. Elevated concentrations in groundwater may indicate influences of human and intensive land use activities.

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.