REVISION AND TESTING OF BMWP
Background to research
The regulatory authorities have used Biological Monitoring Working
Party (BMWP) scores as part of their river invertebrate status
classification system for some time. The BMWP scores were derived
largely by expert opinion and reflect the perceived sensitivity of
river invertebrate families to organic pollution (Hawkes, 1997).
Recently, a more objective approach to allocating BMWP scores was
developed by Walley and Hawkes (1996, 1997 and referred to hereafter as
WH), including the incorporation of abundance data. The
preliminary revised BMWP scores have been widely accepted as better
reflecting the differential sensitivities to the combined effects of
organic and other major forms of pollution.
In order for the regulatory authorities to implement a revised BMWP
system it is necessary for the revised scores to be based on as full a
set of data as possible and to be tested against water quality data to
demonstrate whether it does, indeed, represent an improvement on the
An improved system would mean more accurate and precise representation
of the impacts of pollution on the invertebrate fauna and,
consequently, better informed direction of resources for remediation of
It is also important that the effects of revising the system on the
overall classification of UK rivers is estimated.
Original objectives of
As a result of the tendering process and award of contract the work was
divided into the following revised objectives.
- To derive revised scores for the BMWP taxa (and hence
revised ASPT values for all recorded samples) based on presence only
(PO) of taxa.
- To derive abundance-related (AR) BMWP scores (and hence
revised ASPT values for all recorded samples).
- To derive revised RIVPACS predictions using the new scores
for each site in the compiled agency classification data set using the
RIVPACS reference data set and reference to probability of RIVPACS end
- To test revised scoring systems against each other and
against agency water quality data.
- To re-classify the 2000 EA dataset and the most recent SEPA
and EHS data using the revised systems based on PO and AR scores to
indicate the overall impact on UK river classification.
- To test the revised values for ecological credibility by
distributing new and old scores for agency sites to agency biologists
- To produce a technical report detailing the analyses, an
evaluation of the revised systems in comparison with the existing
Objective 6 was added subsequently. In the event the
derivation of the scores and the testing phases took longer than
anticipated (for reasons which are documented) and the EA agreed to
undertake Objective 7.
- Construction of the project database.
- Derivation and testing of revised PO scores.
- Test sensitivity of PO scores with separate datasets.
- Derive and test AR scores.
- Test PO and AR against BMWP scores.
- Derivation of PO and AR scores for taxa contributing to
composite BMWP families and other non-BMWP taxa
- Analysis and evaluation of the impact of the revised scores
on the classification system.
- Feedback and reporting.
Following the completion of the work outlined above an extension to the
project was carried out in which two methods of deriving revised scores
were repeated using modified scores for two BMWP families.
Some of the analysis in the project was undertaken by CIES at its own
initiative and outside of the contract with SNIFFER. This is
documented in the appropriate places in the text of the report.
Key findings and
recommendations relating to work carried out in the original project
The work described in this report was based on the analysis of sample
data provided by the EA, SEPA and EHS, and the care taken in the
construction of project databases has been described. The
datasets cover the entire United Kingdom and are among the largest ever
used for such analyses, ensuring that the revised scores are reliable.
The methodology for producing revised scores was based on the use of a
statistic (or calibration metric) on which the sensitivities of the
BMWP families were ranked. The ranking of the families based
on the metric was then mapped back to provide a revised BMWP score for
each family by means of a ‘primary list’ of
commonly occurring taxa. The mapping was devised so that the
mean and standard deviation of the BMWP scale was preserved exactly for
the families in the primary list, and preserved approximately for other
Three such metrics were tested in the course of the work: the first was
the overall mean ASPT of sites at which each family occurred; the
second was the overall mean EQI(ASPT) of sites at which each family
occurred, and the third was the overall mean ASPT of sites at which
each family occurred, with the data split into site types beforehand
based on substrate composition as in Walley & Hawkes (1996,
The method based on the use of ASPT alone had significant deficiencies,
in that many of the derived scores for non-primary families, which tend
to occur in pools, were significantly under-estimated. The
likely reason for this is that few of the high scoring families are
present in pools and the consequent low ASPT values translate to low revised
scores. Abundance-related scores were derived using the
approach and distributions similar to Walley & Hawkes (1997)
were found for commonly occurring pollution-sensitive and
pollution-tolerant taxa. Despite this, the ASPT approach is
undermined by the inability to account for site type effects and is not
The use of EQI(ASPT) as the metric does in principle take site type
into account but the scores for the same pool families tended to be
over-estimated. The likely reason for this is the
under-prediction of ASPT by RIVPACS for pool sites due to the lack of
non-impacted pool reference sites used in the calibration of
RIVPACS. Abundance-related scores were derived using this
approach too, but this generated excessively low scores for several of
the commonly occurring pollution-tolerant taxa.
The third approach (termed New WH, or NWH) was based on the original WH
method, in which separate analyses were undertaken by splitting the
data into riffle, riffle/pool and pool sites and an overall score for
each taxon was produced based on the relative frequency of occurrence
within each site type. For the majority of the taxa, the
overall revised score was within ±1 of the original WH
score, with around a dozen exceptions. Abundance-related
scores generally match the original WH values reasonably well for
commonly occurring pollution-sensitive and pollution-tolerant
taxa. A comparison of the scores for each of the three
methods is presented.
A fourth set of analyses generated scores using the NWH approach for a
further 86 taxa made up of the individual taxa comprising the composite
BWMP families and additional taxa for which no score had previously
been allocated. This required data relating to the abundance
levels of the individual families, which was only available in the EA
dataset. Presence-only and abundance-related scores have been
produced on this basis.
Finally, the original BMWP scores were appropriate to the most tolerant
member of the family to organic pollution (and hence the scores are not
ideally suited to analysis via ASPT since they are more akin to lower
bounds than averages). However the revised scores aim to be
more representative of the family as a whole and reflect general
pollution not just organic.
Key findings and recommendations relating to work carried out in the
extension to the project
Evaluation in conjunction with the project board confirmed the
effectiveness of both the EQI(ASPT) and NWH approaches over the ASPT
approach. However, both approaches tended to produce very low
scores for a small number of taxa associated with heavily polluted
waters, especially when occurring in high abundances. An
extension to the project was undertaken in which both approaches were
repeated with increased scores for Oligochaeta and Chironomidae, and
hence modified ASPT values (MASPT).
The results of both the EQI(MASPT) and MNWH (modified NWH) approaches
demonstrated increased scores for the target taxa, especially at high
abundance levels, although additional factors used when generating the
EQI(MASPT) values produced a more complicated response in the scores of
the taxa for that approach than for MNWH. This placed into
question the reliability of the EQI(MASPT) approach.
Presence-only and abundance-related scores have been produced with both
Further evaluation based on regression analysis with the original BMWP
and Pre-BMWP scores (see Section 12) implied a superiority of the MNWH
scores over those generated by EQI(MASPT). Following the
decision of the project board to adopt the MNWH scores, further scores
were produced for non-scoring families. It was proposed that
the MNWH scores be referred to by the name ‘WHPT’,
after past and present contributors Walley, Hawkes, Paisley &
Keywords: river, quality, pollution, classification, monitoriong,
biology, biotope, benthic, macroinvertebrates, biotic indices, abundance
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