The Council surveys macroinvertebrate communities (insects, crustaceans and worms) to assess the general health of our rivers and the impact of wastewater discharges or water abstractions.
Macroinvertebrates vary in their tolerance to pollution. Some invertebrates cannot tolerate any form of pollution and are described as 'sensitive'. Invertebrates that survive or even thrive in polluted habitats are described as 'tolerant'.
A sample of streambed material is collected from a stony riffle area by 'kick-sampling'. Good quality habitats usually have a variety of 'sensitive' invertebrate groups, and large populations. Generally, as the habitat quality reduces, the number of these groups decreases and they are replaced by more 'tolerant' macroinvertebrates.
Macroinvertebrate community index (MCI)
The different types of macroinvertebrates are given sensitivity scores of 1 to 10, according to their organic pollution tolerance. Highly 'sensitive' macroinvertebrates score 9 or 10, while the 'tolerant' forms can score as low as 1. The scores for all the groups in a sample are averaged, and multiplied by 20 to obtain the macroinvertebrate community index (MCI).
Generally, MCI scores in a typical Taranaki ring plain stream are approximately 140 near the boundary of Egmont National Park, and 80 near the coast. A severely polluted habitat may have an MCI as low as 40.
A low MCI may also indicate a poorer habitat, and may be little to do with poor water quality in that habitat e.g. an open slow flowing stream with a mud bed will not support a community as rich as a mountain stream flowing through bush.