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 instream plant life and is commonly measured as either total phosphorus (TP) or dissolved reactive phosphorus (DRP).

Total Phosphorus

Total phosphorus (TP) is a measure of all types of 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.  In this form, it is called dissolved reactive phosphorus (DRP).  DRP concentrations are an indication of a waterbody’s ability to support algae and plant growth.  

DRP is also the mobile form of phosphorus in groundwater, so it is the form most likely to be transported to surface water via groundwater springs. 

The National Policy Statement for Freshwater Management (2020) outlines an attribute band assessment table for DRP in rivers, and for TP in lakes.  There are currently no guidelines or limits set for acceptable concentrations of DRP in groundwater, although some regional councils and unitary authorities have set surface water quality limits for DRP.

Why is too much phosphorus a problem?

Very high phosphorus concentrations in a stream, river or lake are likely to cause rapid weed growth or algal blooms which can choke aquatic life and cause long-term damage to the health of a waterbody.  Where groundwater supports surface water flows, the DRP in the groundwater can also contribute to the growth of algae in surface water.

Where does phosphorus come from?

Phosphorus can occur naturally in rocks and minerals, and it 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 improved 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.  A series of National Environmental Monitoring Standards (NEMS) for water quality were released in 2019, and over time councils will implement these new standards into their water quality monitoring regimes.

Which unit is it given in?

Both TP and DRP can be reported in units of either parts per million (ppm), usually expressed as the equivalent units of g/m3 or mg/L) or parts per billion (ppb). 1 ppb = 0.001 g/m3.

Where do I find more information?

Australian and New Zealand Governments 2018. Deriving guideline values for water quality. Australian and New Zealand guidelines for fresh and marine water quality. Canberra (ACT): ANZG and Australian state and territory governments https://www.waterquality.gov.au/anz-guidelines/guideline-values/derive.

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., (2011). 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. 

National Environmental Monitoring Standards (NEMS) https://www.nems.org.nz/about-nems/