Revision of the Biological Monitoring Working Party (BMWP) Score System: Derivation of Present-only and Abundance-related Scores from Field Data

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 macro-invertebrates. The score allocated to each taxon was set by a group of expert river biologists based upon their collective experience of the taxon’s sensitivity to organic pollution (Hawkes, 1997). This paper describes an objective reappraisal of these subjectively-derived scores through the statistical analysis of a large and comprehensive database of field samples. Using the analytical approach of Walley & Hawkes (1996, 1997) 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, MNWH (Modified New Walley Hawkes), used MASPT (a Modified Average Score Per Taxon) as its ranking metric and required analyses performed on separated data sets divided into three sites types. The other method already accounted for type effects and was based on EQIMASPT, which used the EQI (Environmental Quality Index) based on MASPT to provide its ranking metric. Both of these methods involved the use of modified BMWP scores for Oligochaeta and Chironomidae for reasons that are explained and justified. Following evaluation the investigation proceeded using the MNWH methodology to derive present-only (PO) and abundance-related (AR) scores for the BMWP taxa (and 19 additional taxa mostly from the order Diptera plus 16 extra taxa resulting from the splitting of eight BMWP composite taxa). The MNWH scores are shown to be generally in keeping with Walley & Hawkes (WH) scores, except for a few rare taxa that were represented by very small samples in the Walley & Hawkes study.