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Home > NIEA > Water Home > Water Quality > Rivers > Historical monitoring results > GQA Biological Classification System

The Biological General Quality Assessment Scheme

Last updated: 13 August 2009


Northern Ireland rivers support over 1,500 species of aquatic macroinvertebrates(animals with no backbone and visible to the naked eye) such as insect larvae, molluscs and shrimps which respond to different kinds of  pollutions in different ways..

For example, shrimps and mayfly larvae tend to be sensitive to the effects of acidification, whereas stonefly nymphs are highly sensitive to low dissolved oxygen levels that might result from pollution by organic wastes. Molluscs are sensitive to metal pollution which interferes with their shell forming processes.

Unpolluted waters contain a wide diversity of these organisms, usually with no single species in great abundance. The effect of pollution is to selectively remove certain types of organisms,which can result in other species becoming overly abundant.

For example, the discharge of biodegradable organic matter into a river can result in the disappearence of stonefly nymphs while encouraging the productivity of pollution-insensitive organisms such as the oligochaete worms, midge larvae and hog-lice.

Moreover, when invertebrate communities are damaged by environmental stress, complete recovery can take several months. Macroinvertebrates can therefore act as an in-line monitoring system for pollution events.

Because of their relative lack of mobility in rivers, these organisms are exposed to the full effects of pollution. For these reasons, the monitoring of  the diversity and abundance of macroinvertebrates within a river gives provides a ready means of detecting intermittent pollution and the effects of substances such as pesticides and acids.

Because of the relatively small range of chemical elements routinely monitored, rivers can be classified as of good chemical quality while supporting an impoverished macroinvertebrate community. The effects of pollution as a result can be underestimated if there is over-reliance  on one classification syste.

In the same way, the abundance and diversity of aquatic plants and algae can provide valuable information regarding nutrient enrichment in river waters and sediments.

Assessing biological water quality

Macroinvertebrate data is summarised throughout the UK using the Biological Monitoring Working Party (BMWP) biotic score system. This method of data collation separates invertebrate groups, or taxa,  on the basis of their relative sensitivity to pollution with the more pollution sensitive taxa being allocated higher scores.

The overall community is described by the sum of the individual taxon scores. Generally, the higher the total biotic scores the better the  water quality.

Two other measures which describe biological quality are:

  • the number of BMWP scoring taxa present, and
  • the average pollution sensitivity of the macroinvertebrate community as described by the Average Score per Taxon (ASPT),

Biological classification

Since the late 1970s, a computer model called RIVPACS (River Invertebrate Prediction and Classification System) has been under development in the UK. Using the physical, geographical and chemical characteristics of a monitoring site, RIVPACS can predict what the natural macroinvertebrate fauna of that site would be in the absence of environmental stress of which pollution is an important form.

The computer model was modified prior to the 1995 five year survey to take account of factors that are peculiar to NI. For example, certain macroinvertebrates found in high quality waters in England, Scotland and Wales may be absent from Northern Ireland waters not because the waters are polluted, but because the organisms in question have not colonised Irish waters.

This modification has improved the accuracy of biological water quality classification in Northern Ireland. Further modifications are being carried out to improve the accuracy with which smaller streams and headwaters can be classified.

Comparison of the predicted macroinvertebrate communities with those observed during the biological sampling and analytical programme allows the calculation of ecological quality indices (EQIs). The most relevant EQIs in describing biological quality are those based on the number of macroinvertebrate taxa and on ASPT. These are derived from the equations:

EQI taxa = BMWP observed number of taxa
BMWP predicted number of taxa from RIVPACS


BMWP predicted ASPT from RIVPACS

An EQI value of approximately one indicates that the observed macroinvertebrate fauna is what would be expected in an unstressed river reach, whereas lower EQI values reflect communities that are stressed to a lesser or greater degree. The EQI bandings agreed nationally for the range of biological qualities are set out in Table 1.

Table 1: Biological Classification Bandings

Biological Class EQI for ASPT EQI for Taxa
A (Very Good) 1.00 or above 0.85 or above
B (Good) 0.90-0.99 0.70-0.84
C (Fairly Good) 0.77-0.89 0.55-0.69
D (Fair) 0.65-0.76 0.45-0.54
E (Poor) 0.50-0.64 0.30-0.44
F (Bad) less than 0.50 less than 0.30

Class A – Very Good

The biology is similar to (or better than) that expected for an average, unpolluted river of this size, type and location. There is a high diversity of taxa, usually with several species in each. It is rare to find a dominance of any one taxon.

Class B – Good

The biology shows minor differences from Class A and falls a little short of that expected for an unpolluted river of this size, type and location. There may be a small reduction in the number of taxa that are sensitive to pollution, and a moderate increase in the number of individuals in the taxa that tolerate pollution (like worms and midges). This may indicate the first signs of organic pollution.

Class C - Fairly Good

The biology is worse than that expected for an unpolluted river of this size, type and location. Many of the sensitive taxa are absent or the number of individuals is reduced, and in many cases there is a marked rise in the numbers of individuals in the taxa that tolerate pollution.

Class D – Fair

The biology shows considerable differences from that expected for an unpolluted river of this size, type and location. Sensitive taxa are scarce and contain only small numbers of individuals. There may be a range of those taxa that tolerate pollution and some of these may have high numbers of individuals.

Class E – Poor

The biology is restricted to animals that tolerate pollution with some taxa dominant in terms of the numbers of individuals. Sensitive taxa will be rare or absent.

Class F - Bad

The biology is limited to a small number of very tolerant taxa, often only worms, midge larvae, leeches and the water hog-louse. These may be present in very high numbers but even these may be missing if the pollution is toxic. In the very worst case there may be no life present in the river.