Geographic pattern of phosphorus in river water

  • Image, Geographic pattern of phosphorus in river water.

    Phosphorus is an essential nutrient for plants, and small amounts are a natural component of healthy rivers. Agricultural and urban land use can add more phosphorus to waterways, leading to excessive growth of algae, which can degrade river habitats. We report on two measures of phosphorus: total phosphorus and dissolved reactive phosphorus. Levels of phosphorus in New Zealand rivers vary as a result of differences in land use, climate, and geology.

    We classified Geographic pattern of phosphorus in river water as a case study.

    Key findings

    Over the 2009–13 period, median concentrations of total phosphorus were highest at sites where the upstream catchment’s dominant land cover was urban (47.7 mg/m3). This compares with catchments where the dominant land cover was pastoral (32.1 mg/m3), exotic forest (24.2 mg/m3), or indigenous (10.0 mg/m3).

    • Median concentrations of dissolved reactive phosphorus were highest at sites where the dominant catchment land cover was exotic forest (19.0 mg/m3). This compares with catchments where the dominant land cover was urban (18.5 mg/m3), pastoral (13.5 mg/m3), or indigenous (5.5 mg/m3).
    • The exotic forest sites were mostly located in catchments fed by the Volcanic Plateau in the central North Island, where levels are naturally high due to phosphorus-rich geology (Timperley, 1983).

    Figure 1

    Dissolved Reactive Phosphorus (DRP) median and trends – interactive map

    Figure 2

    Total Phosphorus median and trends – interactive map

    Figure 3

    Note: 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 maximum values for urban and pastoral are 410 mg/m3 and 338 mg/m3, respectively (which are beyond the range of this figure). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

    Figure 4

    Note: 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 maximum values for urban and pastoral are 270 mg/m3 and 290 mg/m3, respectively (which are beyond the range of this figure). The blue line (at 33.2 mg/m3) represents the median dissolved phosphate threshold (Larned et al, 2015) required to meet the periphyton minimum standard in the National Policy Statement for Freshwater Management 2014. The threshold varies with climate and the source of flow (Larned et al, 2015). Nationally, the proportion of river length classified as predominantly urban is 0.8 percent; pastoral, 45.8 percent; indigenous forest, 47.7 percent; and exotic forest, 5.6 percent.

    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, the phosphorus is not available to plants and algae as a nutrient. However, over time and in the right conditions it can gradually dissolve into the river water, stimulating the growth of aquatic algae for many years after the sediment has been deposited.

    We report on two measures of phosphorus:

    • total phosphorus, accounting for all the phosphorus in rivers regardless of its form. This includes the portion dissolved and readily available to plants and algae, and the portion bound to soil or sediment that may become available in the future
    • dissolved reactive phosphorus, indicating how much phosphorus is immediately available to support algae and plant growth.

    River water samples are collected at fixed locations and sent to a laboratory for chemical analysis.

    Data are from 577 river sites for total phosphorus and 519 river sites for dissolved reactive phosphorus, monitored monthly or quarterly by NIWA and the 16 regional councils.

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

    About 48.4 percent of New Zealand’s river length is fed by catchments that are mainly influenced by indigenous land cover, while 45.7 percent are from catchments influenced mainly by pasture, 5.1 percent by exotic forest, and 0.8 percent by urban land cover.

    Regional councils monitor river water quality to manage environmental impacts. These sites tend to be in catchments dominated by agricultural land use. Rivers in most areas, particularly 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 well represented.

    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.

    Data quality

    We classified Geographic pattern of phosphorus in river water as a case study.

    Relevance

    relevance-partial This case study is a partial measure of the ‘Chemical properties of river water, lake water, and groundwater’ topic.

    Accuracy

     The accuracy of the data source is of medium quality.

    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. Journal of Hydrology (NZ), 44(1), 1–27.

    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.

     

    Published 21 October 2015

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