Predicted average Macroinvertebrate Community Index (MCI) score, 2007–2011

Source:
Cawthron Institute

Method:
"Macroinvertebrates are measured by disturbing or sweeping a small representative area of river bank and bed with a net, and sieving out the macroinvertebrates. They are then identified and counted under a microscope.
Estimates of median MCI scores across New Zealand is based on annual measurements from 436 river sites monitored by the 16 regional councils and 77 sites along 35 major rivers measured monthly by the National Institute of Water and Atmospheric Research (NIWA).
The MCI involves scoring the diversity of taxa observed at a site based on their tolerance to pollution. Those taxa which are characteristic of more unpolluted conditions score more highly than those that dominate in polluted streams, and contribute to a higher MCI score. In this way, higher MCI scores generally indicate better river condition. The following MCI scores are a guide for interpreting the river health classes:
- Excellent (MCI score greater than 119)
- Good (MCI score equal to or greater than 100 to 119)
- Fair (MCI score equal to or greater than 80 to 99)
- Poor (MCI score less than 80)
MCI scores alone do not account fully for natural variation in stream types. Even under natural conditions, some stream types do not attain scores high enough to achieve an 'excellent' quality rating due to natural factors such as climate or lack of suitable riverbed substrate.
This is inferred from the predominant land cover in a catchment and the surrounding landscape characteristics, such as, climate, elevation, and geology.
The accuracy of the data source is of medium quality."