River water quality: macroinvertebrate community index

  • Image, River water quality: benthic macroinvertebrates.

    Benthic macroinvertebrates are small animals without backbones (eg insects and worms). They live on and under submerged logs, rocks, and aquatic plants on the beds of rivers and streams during some part of their life cycle. Macroinvertebrates play a central role in stream ecosystems by feeding on periphyton (algae), macrophytes (aquatic plants), dead leaves and wood, or on each other. In turn, they are an important food source for fish and birds. We report on the states and trends in the macroinvertebrate community index (MCI) as an indicator of water quality and overall stream health. A high MCI indicates a high level of river health, with more polluted rivers having low MCI scores.

    We classified River water quality: macroinvertebrate community index as a case study.

    Key findings

    Median MCI tended to be highest at sites in the native land-cover class, compared with sites in the exotic forest, pastoral, or urban land-cover classes from 2009 to 2013.

    States, for the period 2009–13: 

    • median MCI was excellent or good at 62.9 percent, fair at 26.4 percent, and poor at 10.7 percent of monitored sites 
    • model-based predictions of MCI showed 76.6 percent of river length were excellent or good, 22.3 percent were fair, and 1.1 percent were poor.

    Trends, for the period 2004–13, at sites in the pastoral land-cover class (most monitored sites were pastoral): 

    • MCI improved at 4.5 percent of sites and worsened at 12.8 percent of sites 
    • a trend in MCI could not be confidently determined for 82.7 percent of sites due to insufficient data.

    Figure 1

    Note: Sites are classified by dominant land cover in the upstream catchment . Higher MCI scores are better than lower scores, so data below the dashed line are considered ‘poor’. 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 represents the ‘poor’ threshold (Stark & Maxted, 2007).

    Figure 2

    Note: Macroinvertebrate community index (MCI) are for 512 monitored sites.

    Figure 3  

    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 4

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

    Definition and methodology

    Macroinvertebrates are measured by disturbing or sweeping a small representative area of riverbank and bed with a net, and sieving out the macroinvertebrates (Stark & Maxted, 2007). They are then identified under a microscope. Sampling is normally conducted annually.

    Macroinvertebrates are relatively stable, continuous indicators of the health of their stream environment. This is because they are relatively sedentary and long-lived (surviving for a year or longer), and respond to multiple stresses or changes in their location (eg pollution, habitat removal, floods, and droughts).

    The MCI assigns ‘scores’ to taxa (types of macroinvertebrates) based on their tolerance to pollution. Scores for each taxa found at a site are combined and an overall MCI score is calculated: excellent (>119); good (100–119); fair (80–99) and poor (<80). Taxa that are characteristic of less polluted conditions score more highly than those that dominate polluted streams. A higher MCI generally indicates better river health.

    On its own, the MCI does not fully account for natural variation in river or stream types. Even under natural conditions, some waterway types do not achieve an 'excellent' quality rating. This can be due to natural factors, such as climate or lack of suitable riverbed substrate.

    For the analysis presented here, NIWA used MCI data from up to 512 river  sites monitored by them and the 16 regional councils with consistent time periods and comparable methods (Larned et al, 2015). Regional council data from 461 sites  were used to assess trends over a 10-year period. Increasing and decreasing trends at sites are inferred with 95 percent confidence using the Relative Seasonal Sen Slope Estimator. Indeterminate trends mean we have insufficient data to determine trend direction.

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

    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.

    NIWA also modelled current state in MCI for rivers using data monitored by them and regional councils (Larned et al, 2016). Median MCI for 2009–13 was predicted for all river segments using Random Forest modelling and predictors (explanatory variables) from the Freshwater Ecosystems of New Zealand database. The model performed well  with low bias (R2 of 0.69).

    Data quality 

    Topic

     Measure

    Classification 

     Relevance

    Accuracy 

     Freshwater quality, quantity and flows

    Modelled river water quality: macroinvertebrate community index



    Case study

    relevance-partial 
    Partial

     Image, Medium accuracy.
    Medium

    Monitored river water quality: macroinvertebrate community index



    Case study

    relevance-partial 
    Partial 

    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. 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. Wellington: NIWA. Retrieved from 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.

    Stark, J & Maxted, J (2007). A user guide for the macroinvertebrate community index. Cawthron Report no. 1166. Retrieved from www.mfe.govt.nz.

    Archived pages

    See River water quality: benthic macroinvertebrates (archived April 2017).

    Updated 27 April 2017

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