River water quality: nitrogen

  • Image, River water quality tredns: nitrogen.

    Nitrogen is an essential nutrient for plants. Small amounts are a natural component of healthy rivers, but agricultural and urban land use can add more nitrogen to waterways. We report on the states and trends in three indicators of nitrogen in rivers: total nitrogen, nitrate-nitrogen and ammoniacal nitrogen. Too much total nitrogen and nitrate-nitrogen can lead to excessive growth of algae, which can deteriorate river habitats. Nitrate-nitrogen and ammoniacal nitrogen can be toxic to aquatic life.

    We classified River water quality: nitrogen as a case study.

    Key Findings

    Median nitrate-nitrogen, total nitrogen, and ammoniacal nitrogen concentrations were higher at sites in the urban land-cover class, compared with sites in the pastoral, exotic forest, or native land cover classes from 2009 to 2013.

    States, assessed using model-based estimations over the five-year period 2009–13:

    • nitrate-nitrogen concentrations at 31.8 percent of river length did not meet the Australian and New Zealand Guidelines for Fresh and Marine Water Quality (ANZECC) trigger values for slightly disturbed upland ecosystems (upland and lowland)
    • total nitrogen concentrations at 39.4 percent of river length did not meet the ANZECC triggers value for slightly disturbed ecosystems (upland and lowland)
    • ammoniacal nitrogen concentrations at 33.6 percent of river length did not meet the ANZECC trigger values for slightly disturbed ecosystems (upland and lowland)
    • less than 1 percent of river length did not meet the national bottom line for nitrate and ammonia toxicity.

    Nationally, nitrate-nitrogen concentrations worsened at 55.3 percent of all monitored sites and improved at 27.6 percent of sites for the 20-year period 1994–2013. Of these monitored sites, the majority (68.1 percent) are in the pastoral land-cover class.

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

    • nitrate-nitrogen concentrations worsened at 60.6 percent of sites and improved at 22.3 percent of sites
    • total nitrogen concentrations worsened at 51.2 percent of sites and improved at 17.0 percent of sites
    • ammoniacal nitrogen concentrations improved at 58.8 percent of sites and worsened at 12.9 percent of sites.

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

    • Ammoniacal nitrogen concentrations improved at more sites than worsened.
    • Total nitrogen and nitrate-nitrogen concentrations worsened at more sites than improved.

    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 bottom dashed line at 444 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 value for lowland streams was chosen as most of the sites are in lowland areas (the trigger value for upland streams is 167 mg/m3). One pastoral site exceeded the national bottom line (6,900 mg/m3; line not shown) for the nitrate-nitrogen level. This means impacts on growth and mortality of multiple sensitive species are expected above this level (National Policy Statement for Freshwater Management 2014).

    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 614 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 value for lowland streams was chosen as most of the sites are in lowland areas (the trigger value for upland streams is 295 mg/m3).

    Figure 3

    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 1,300 mg/m3) represents the national bottom line for the ammoniacal nitrogen level . This means impacts on growth and mortality of multiple sensitive species are expected above this level  (National Policy Statement for Freshwater Management 2014).

    Figure 4

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

    Figure 5

    Figure 6

    Note: The National Objectives Framework (NOF) bands in the National Policy Statement for Freshwater Management 2014 show the concentrations where nitrate-nitrogen and ammoniacal nitrogen level are expected to impact on growth and mortality of multiple species. Bands A through D represent different states, with A being the best state and D the worst. The national bottom line is the boundary between bands C and D, which means that impacts on growth and mortality of multiple sensitive species are expected above this level.

    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.

    Figure 9

    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 10

    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 11

    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 12

    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

    We report on the following indicators of nitrogen in the environment:

    • total nitrogen, which is the sum of all nitrogen forms found in a river water sample, including organic nitrogen from living and dead organic material 
    • nitrate-nitrogen, which is highly soluble (dissolves in water) and can be readily used by plants and algae to help grow; it can also easily leach (drain) through soils, particularly sandy soils or after heavy rainfall (McDowell et al, 2008) 
    • ammoniacal nitrogen (can form ammonia in water under certain conditions), which can be toxic to aquatic life; elevated quantities in waterways are primarily from direct discharges of pollutants such as untreated effluent; like nitrate-nitrogen, ammoniacal nitrogen is also readily used by plants and algae to grow.

    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 analysis presented here, NIWA used nitrogen data from up to 354 to 587 sites (depending on the nitrogen indicator) 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 nitrogen, nitrate-nitrogen, and ammoniacal nitrogen over the period 1994–2013.

    Table 1

    Number of river sites measured for states and trends in total nitrogen, nitrate-nitrogen, and ammoniacal nitrogen, 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 nitrogen

    354

    243

    91

     Nitrate-nitrogen

    587

    511

    257

     Ammoniancal nitrogen

    365

    206

    138

    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 the current state for three nitrogen indicators at rivers using data monitored by them and regional councils (Larned et al, 2016). Median total nitrogen, nitrate-nitrogen, and ammoniacal nitrogen concentrations for 2009–13 were predicted 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 nitrogen R2 of 0.76, nitrate-nitrogen R2 of 0.71, and ammoniacal nitrogen R2 of 0.51).

    Data quality

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

     relevance-direct
    Direct

    Image, Medium accuracy.

    Medium 

    Monitored river water quality: nitrogen  Case study

     relevance-direct
    Direct

     

    Image, Medium accuracy.

    Medium 

     

     See Data quality information for more detail.

    References

    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. Wellington: NIWA. 

    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.

    McDowell, RW, Houlbrooke, DJ, Muirhead, RW, Müller, K, Shepherd, M, & Cuttle, SP (2008). Grazed pastures and surface water quality. New York: Nova Science Publishers.

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

    Archived pages

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

    Updated 27 April 2017

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