Revision of the Biological Monitoring Working Party (Bmwp) Score System: Derivation of Present‐only and Abundance‐related Scores From Field Data

ABSTRACT The Biological Monitoring Working Party (BMWP) score system was introduced in 1980 to provide an index of river water quality for England and Wales based on aquatic macroinvertebrates. The score allocated to each taxon was set by a group of expert river biologists on the basis of their collective experience of the taxon's sensitivity to organic pollution. This paper describes an objective reappraisal of these subjectively derived scores by the statistical analysis of a large and comprehensive database of field data. Two different methods of using biological and environmental data were evaluated as a basis for revising BMWP scores, both of which accounted for variations in site type. One method, Modified New Walley–Hawkes (MNWH), used a modified average score per taxon (MASPT) as its ranking metric and required analyses performed on separated data sets divided into three site types. The other method used the environmental quality index based on MASPT (EQI MASPT ) to provide its ranking metric. Both of these methods involved the use of modified BMWP values for Oligochaeta and Chironomidae for reasons that are explained. Following evaluation, the MNWH method was chosen to derive present‐only and abundance‐related scores for the BMWP taxa, 19 additional taxa mostly from the order Diptera, and 16 extra taxa resulting from the splitting of eight BMWP composite taxa. The MNWH scores were generally in keeping with those derived in an earlier study, except for a few rare taxa that were represented by few samples in that study. The UK Regulatory Authorities intend adopting a new index based on the MNWH scores, and this is given in two appendices. Copyright © 2013 John Wiley & Sons, Ltd.