River water quality: phosphorus

  • Image, River water quality tredns: phosphorus.

    Phosphorus is an essential nutrient for plants and is a natural component of healthy rivers. Agricultural and urban land use can add more phosphorus to waterways, mainly as attached to sediment, which can cause excessive growth of algae and deteriorate river habitats. We report on the states and trends for two indicators of phosphorus in rivers: total phosphorus and dissolved reactive phosphorus. Dissolved reactive phosphorus is the form of phosphorus immediately available to support plant and algae growth.

    We classified River water quality trends: phosphorus as a case study.

    Key findings

    Median dissolved reactive phosphorus concentrations were highest at sites in the exotic forest land-cover class, compared to sites in the urban, pastoral, and native land-cover classes from 2009 to 2013. The exotic forest sites were mostly located in catchments fed by the Volcanic Plateau in the central North Island, where concentrations tend to be naturally high due to phosphorus-rich geology (Timperley, 1983).

    States for the five-year period 2009–13: 

    • median total phosphorus concentrations were highest at sites in the urban land cover class, compared with sites in the pastoral, exotic forest, or native land-cover classes 
    • model-based estimations of dissolved reactive phosphorus concentrations show that 67.0 percent of river length did not meet the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) trigger values (upland and lowland) for slightly disturbed upland ecosystems 
    • model-based estimations of total phosphorus concentrations show that 34.2 percent of river length did not meet the ANZECC trigger values (upland and lowland) for slightly disturbed ecosystems.

    Nationally, phosphorus concentrations improved at 42.1 percent of all monitored sites and worsened at 24.9 percent of sites for the 20-year period 1994–2013. Of these monitored sites, the majority (69.4 percent) are in the pastoral land-cover class.

    Trends for the 20-year period 1994–2013, at sites in the pastoral land-cover class:

    • dissolved reactive phosphorus concentrations improved at 45.5 percent of sites and worsened at 20.7 percent of sites
    • total phosphorus concentrations improved at 40.4 percent of sites and worsened at 21.1 percent. 

    Trends for the 10-year period 2004–13, at sites in the pastoral land-cover class:

    • dissolved reactive phosphorus concentrations improved at 56.7 percent of sites and worsened at 14.8 percent of sites
    • total phosphorus concentrations improved at 64.5 percent of sites and worsened at 5.6 percent of sites.

    Figure 1

    Note: Sites are classified by dominant land cover in the upstream catchment. The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The dashed line at 33 mg/m3 represents the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) recommended trigger value for physical and chemical stressors for slightly disturbed lowland streams. The ANZECC value for lowland streams was chosen as most of the sites are in lowland areas (the trigger value for upland streams is 26 mg/m3).

    Figure 2

    Note: Sites are classified by dominant land cover in the upstream catchment. The ends of each ‘box’ in the box-plot are the upper and lower quartiles (25 percent of the sites are either higher or lower than these values). The top and bottom ‘whiskers’ represent the highest and lowest value. The middle line of the box represents the median (middle) data point (half the sites are above and half below this value). The dashed line at 10 mg/m3 represents the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) recommended trigger value for physical and chemical stressors for slightly disturbed lowland streams. The ANZECC value for lowland streams was chosen as most of the sites are in lowland areas (the trigger value for upland streams is 9 mg/m3).

    Figure 3

    Note: Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) recommend default trigger values for dissolved reactive phosphorus (DRP) and total phosphorus (TP) indicative of unmodified or slightly disturbed ecosystems in New Zealand.  Trigger values are ‘warning’ values where TP or DRP concentrations are at a threshold that may require an investigation or a management action put in place. For total TP and total DRP, the number of exceedances for upland and lowland segments were calculated separately and then added together.

    Figure 4

    18 April 2019: The interactive map will be available again when it has been updated.

    Figure 5

    Note: Sites are classified by land-cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 6

    Note: Sites are classified by land-cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 7

    Note: Sites are classified by land-cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Figure 8

    Note: Sites are classified by land-cover class. The land-cover class assigned to a site is based on the land cover in the upstream catchment that is presumed to dominate conditions in surface water.

    Definition and methodology

    Most phosphorus enters New Zealand rivers attached to eroded soil (Elliot et al, 2005), which settles on riverbeds. While it is attached to this sediment, the phosphorus is not immediately available as a nutrient for plants and algae. However, over time and in the right conditions it can be released from the sediment, stimulating the growth of aquatic algae for some time after the sediment has been deposited.

    We report on two measures of phosphorus.

    • Total phosphorus, accounting for all phosphorus in New Zealand rivers regardless of its form. This includes both the portion dissolved and readily available to plants and algae, and the portion bound to soil or sediment, or in organic forms, that may become available in the future.
    • Dissolved reactive phosphorus, which indicates how much phosphorus is immediately available to support algae and plant growth.

    Regional councils monitor river water quality to manage environmental impacts. These sites tend to be in catchments dominated by agricultural land use. Rivers in low-lying and hilly areas in the North and South islands are well represented, while mountainous areas in the South Island and parts of the central North Island are not.

    For the data presented here, NIWA used total phosphorus data from up to 577 sites, and dissolved reactive phosphorus data from up to 519 sites monitored by them and regional councils with consistent time periods and comparable methods (Larned et al, 2015).

    If you want detailed regional-level information, we recommend you review the relevant regional council’s environmental reports.

    This is because although our data are sourced from regional councils, we adjust some datasets to ensure our reports are nationally consistent. The adjustments may include omitting information produced by non-comparable methods. As a result, our evaluations may differ from those produced by regional councils.

    Table 1 shows the number of river sites measured for total phosphorus and dissolved reactive phosphorus over the period 1994–2013.

    Table 1

    Number of river sites measured for states and trends in total phosphorus and dissolved reactive phosphorus concentrations, 1994–2013
     Measure

    State of water quality using five-year median

    Trends in water quality

    No. sites measured

    2009–13

    2004–13

    1994–2013

     Total phosphorus

    577

    421

    231

     Dissolved reactive phosphorus

    519

    391

    209

    Note: Trend data were are adjusted for the influence of variation in river flow, because values are strongly correlated with flow. Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean there was not enough data to determine a trend (Larned et al, 2015).

    Sites are classified by land cover using the River Environment Classification (Snelder & Biggs, 2002).

    NIWA also modelled current state for dissolved reactive phosphorus and total phosphorus concentrations in rivers, using monitored data from NIWA and regional councils (Larned et al, 2016). NIWA estimated median total phosphorus and dissolved reactive phosphorus concentrations for 2009–13 for all river segments using Random Forest modelling and predictors (explanatory variables) from the Freshwater Ecosystems of New Zealand database. The models for all indicators performed well, with low bias (total phosphorus R2 of 0.67 and dissolved reactive phosphorus R2 of 0.58).

    Data quality

    Topic  Measure Classification   Relevance Accuracy 
     Freshwater quality, quantity and flows Modelled river water quality: phosphorus  Case study

     relevance-direct
    Direct

     

    Image, Medium accuracy.

    Medium 

     

    Monitored river water quality: phosphorus  Case study

     relevance-direct
    Direct

     

    Image, Medium accuracy.

    Medium

     

     

    See Data quality information for more detail.

    References

    Elliott, AH, Alexander, RB, Schwartz, GE, Shanker, U, Sukias, JPS, & McBride, GB (2005). Estimation of nutrient sources and transport for New Zealand using the hybrid mechanistic-statistical model SPARROW (PDF, 1.41MB). Journal of Hydrology New Zealand, 44(1), 1–27. Retrieved from www.niwa.co.nz.

    Larned, S, Snelder, T, & Unwin, M (2017). Water quality in New Zealand rivers: Modelled water quality state. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2016-070. Retrieved from www.mfe.govt.nz.

    Larned, S, Snelder, T, Unwin, M, McBride, G, Verburg, P, & McMillan, H (2015). Analysis of water quality in New Zealand lakes and rivers: Data sources, data sets, assumptions, limitations, methods and results. Prepared for the Ministry for the Environment. NIWA Client Report no. CHC2015-033. Retrieved from https://data.mfe.govt.nz.

    Snelder, T & Biggs, B (2002). Multiscale river environment classification for water resources management. Journal of the American Water Resources Association, 38(5), 1225–1239.

    Timperley, MH (1983). Phosphorus in spring waters of the Taupo Volcanic Zone, North Island, New Zealand. Chemical Geology, 38(3/4), 287–306. 

    Archived pages

    See River water quality trends: phosphorus and Geographic pattern of phosphorus in river water (archived April 2017).

    Updated 18 April 2019

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