Total phosphorus (TP) is a measure of phosphorus in its various forms that occur in New Zealand waterways. It includes the phosphate that is stuck to sediment as well as dissolved reactive phosphorus, which is more readily available for plants. TP is an important measure because most phosphate enters our rivers bound to sediment via run-off.

If you want to find out more about phosphorus, have a look at the phosphorus factsheet.

Current state

TP concentrations across the whole country are estimated using models, which are based on data collected from hundreds of monitoring sites over a five year period to 2012. View a factsheet on how the national picture current state is calculated.

TP concentrations vary across the country, as shown in the map below. Higher total phosphorus concentrations are widespread in the mid- to upper-North Island. In the South Island, the monitoring data indicates higher total phosphorus concentrations exist at sites along the east coast and southern parts of the Island:

Average total phosphorus concentrations

Site Data

This map shows the site locations and value of the monitoring data collected by New Zealand’s councils, which was used to train the model.

Modelled Data

The map illustrates the modelled condition across the country from highest to lowest. The dots indicate the site location and value of the monitoring data, which was used to set up the model.

Comparison Data

The map shows a comparison between modelled concentrations and actual monitoring site data. Symbols show where the site data was substantially (50%) better than the model predicts, or substantially (50%) worse.

Modelled national total phosphorus concentrations

The box plot below shows the modelled total phosphorus results for four different land-cover classes. The data shows that the rivers that drain urban land use have five times the level of phosphorus than a typical reach draining predominantly indigenous land-cover.

Rivers draining pastoral land use have 3.5 times, and rivers that have their catchment in exotic forest, 2.5 times the levels of those in catchments associated with predominantly indigenous land-cover. Rivers and streams draining pasture catchments have the widest range of concentrations (which is shown by the ‘whiskers’ of the pasture box-plot), ranging from the highest in the country to some of the lowest. This might be due to different soil types, different levels of erosion, combined with a range of land-use types and intensities associated with different farming practises:  

Modelled national total phosphorus concentrations for four different land-cover classes 

 TP State boxplot

What is the trigger value? 
What is this type of graph? 

Recent trends

Overall, improving trends were detected at 30% of monitoring sites, outweighing those that show a deteriorating trend (10%), across all land-cover types. This may reflect improving environmental management practices, such as wastewater treatment and riparian planting.

The graph below shows that total phosphorus concentrations improved in urban river sites (i.e. TP concentrations decreased). Urban rivers make up less than 1% of the total length of rivers nationwide, indicated graphically by the small size of the column in the graph below. For all other land covers, total phosphorus concentrations showed no evidence of a trend, or decreased (i.e., an improving water quality trend). It is interesting to see that although total phosphorus levels mostly improved in indigenous vegetation, there were also a small proportion of monitoring sites that showed a deteriorating trend (i.e., phosphorus levels increased).

View a factsheet on how national trends are calculated.

Percentage of monitoring sites with recent trends in total phosphorus concentration across four land-cover classes

TP percentage across land cover classes

What is this type of graph?

See also:

– Other nutrients: Dissolved reactive phosphorus and nitrate-nitrogen
Benthic macroinvertebrates
Faecal indicators (E. coli)