Comparisons of Irish River Water Quality Indices

Niger Delta. Source: NASA

River and stream quality may be measured by assessing the diversity of macroinvertebrates supported by the water body being surveyed (Sutherland et al., 2006). For this purpose, different biotic indices have been devised. These assign values to different biological parameters, and can sometimes take abiotic factors into account, such as siltation and dissolved oxygen. The main component of aquatic biotic indices is the identification of macroinvertebrates collected from the surveyed water body. These invertebrates act as indicator species, in that their presence and abundance indicate the quality of the water, usually due to the varying sensitivity of individual families, and sometimes individual species, with differences in tolerance to environmental impacts (Dufrene et al., 1997). By assessing the number of families present, each of which are usually assigned a value, and their abundance, the quality of the water body is given a score, indicating its quality.

The Q-value system used by the Irish Environmental Protection Agency was designed by Dr Paul Toner of An Foras Forbartha (later superseded by the EPA) in the early 1970s (Flanagan et al., 1972). An overview of the system is included in the appendices of several annual EPA reports entitled ‘Biological Survey of River Water Quality’, for example Clabby et al. (2004) and Lucey (2009). Since its development, the Q-value system has been the standard biotic index system used by the EPA to monitor the ecological quality of Irish sreams and rivers (Lucey, 2009). The Q-value system was intercalibrated with the European Union’s Ecological Quality Ratios (EQRs) as per the Water Framework Directive (Lucey, 2009). The WFD requires that biotic indices calculated by each Member State with their own systems be converted to a standardised ecological quality ratio (EQR). EQRs are the ratio between the observed and the reference conditions for the water body being surveyed and are expressed as a number between zero and one, with values close to one representing high ecological status values close to zero representing bad ecological status (McGarrigle et al., 2009).

The Biological Monitoring Working Party (BMWP) system is the main biotic index used in the UK and was first devised in 1976 (Hawkes, 1997), and has been updated a number of times, most recently in 2010 (Paisley et al., 2010).

Other indices that exist include the Trent Biotic Index, the Chandler Biotic Index, the Saprobic Index and the Hilsenhoff Biotic Index.

            Both the BMWP and the Q-value system score stream and river quality based on macroinvertebrate identification and weight families by both presence and abundance; both require habitat information, although in the BMWP system this is limited to river microhabitat type: riffles, pools and glides. The main differences between the systems are the numbers of macroinvertebrate families to which scores are assigned (more in the BMWP system), and the scoring scales themselves (narrower in the Q-value system).

Overall, while the Q-value system has the advantage of including more non-invertebrate parameters in its scoring calculations, the range of assessed invertebrates (the main function of both biotic indices) is much narrower in the Q-value system than in the BMWP system. The small number of bands (Q1-Q5) and the room for overlap between them also mean that this system is less accurate than the BMWP system.

A disadvantage of both biotic indices is the effect that sampling effort has on the final scores of surveyed water bodies. For instance, a higher score may be assigned if the sampling period is extended, and a lower score will be the result of reduced sampling times. However, the BMWP system has the advantage that it incorporates the Average Score Per Taxa (ASPT) which may be calculated by dividing the final BMWP score by the number of taxa (Hawkes, 1997).

           Good or moderate water quality may exist where faunal requirements are not met, for instance in cases where the water is either oligotrophic, very hard and calcareous, or where there is significant groundwater input (Lucey, 2009). In these cases, both biotic indices would fail to assign good quality status to water bodies which are of good physic-chemical quality, but that do not support diverse macroinvertebrate communities. Biotic indices that record macroinvertebrates also exclude some keystone species, with Margaritifera margaritifera being an example of an indicator of good water quality.

            Both the BMWP and the Q-value system test the sensitivity of a range of invertebrate species to changes in their environment. However, this is not the only measure of good water quality, since physico-chemical parameters must also be assessed, and biotic indices should not be relied upon as the sole method to test stream and river quality.

References

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