Phosphorus is an element found naturally in water and soil. Like nitrogen, phosphorus is essential for plant growth. The quantities of phosphorus in soil are generally small, and this can limit plant growth, which is why phosphate fertilisers are often applied on productive land.
When too much fertiliser (commercial fertilisers or manure) is applied to land, phosphorus not used by plants can be lost from the soil through leaching and run-off or overland flow. Because phosphorus usually clings to soil particles, the main way in which phosphorus gets into water is when soil is washed in and becomes sediment. Excess phosphorus causes excessive growth of plants in waterways, lakes and estuaries leading to eutrophication.
Much of the phosphorus in rivers and lakes is a legacy of erosion caused by land development from activities like forest clearance and fertilising for farming. Most New Zealand soils are naturally low in phosphorus, but when washed into water add to the cumulative effect of decades of erosion and topdressing with superphosphate.
Some overflows from wastewater systems from our towns and factories, as well as poorly located or maintained septic tanks also contribute to phosphorus in waterways. Household detergents are also a source of phosphorus.
Effects on people and the environment
Excess phosphorus in waterways can cause damage to ecosystems by promoting the overgrowth of large plants called macrophytes, and tiny plants called periphyton and phytoplankton.
The result is that excessive plant growth can lead to dramatic drops in oxygen levels at night, leaving fish and other aquatic creatures unable to breathe.
Excess phosphorus in waterways can contribute to the growth of toxic algae which can make swimming and shellfish gathering spots unsafe. The algae can produce toxins (called cyanotoxins) that are harmful to animals and humans when eaten, even licked, or when water containing the toxins is swallowed. In addition to monitoring freshwater sites around Southland for toxic algae, Environment Southland also monitors popular swimming spots from December through to March, so that the public can be alerted if there is a toxic algae bloom. Check out the latest summer swimming results on the Land, Air, Water Aotearoa website – www.lawa.org.nz
Phosphorus science
We test monthly for the two forms of phosphorus, dissolved reactive phosphorus (DRP) and total phosphorus (TP) at groundwater bores and surface water (rivers, streams, lakes) sites around Southland.
DRP is the amount of phosphorus that has dissolved in water and is therefore readily available for plant growth. TP is the total amount of phosphorus present in water. It includes the phosphorus that is stuck to sediment as well as DRP. TP is a particularly important measure for lakes, because over time phosphorus that is stuck to sediment can be released and become available for plant growth.
Environment Southland has completed scientific modelling, using the latest information and data, to better understand the gap between where our water quality is now and where it needs to be to meet our goals for improved freshwater quality - the Murihiku Southland draft freshwater objectives.
Several modelling studies have been undertaken to predict (estimate) the size of load reductions needed to achieve the objectives related to Total Phosphorus.
The estimated reduction in phosphorus load to achieve just the national bottom lines is 21%. To achieve the water quality standards detailed in the proposed Southland Water and Land Plan the reduction is 69%, and to achieve the draft Murihiku Southland freshwater objectives it is 70%.
Read the report here > Report - Nutrient load reduction modelling - lakes, rivers, estuaries (LWP) (PDF, 3.3MB)
Find out more about this work and our programme to achieve the improvements in water quality needed.
There are a number of things you can do to take action. Find out more about what you can do.
