Page 142 - Freshwater-Biology-and-Ecology-Handbook

CHAPTER 3 2.1 2

METRICS AND

INDICES

2.1 Introduction

The field and laboratory analysis of invertebrate samples results
in a long list of species or other taxa found in the samples together
with their abundances. Whilst an experienced biologist can
interpret much about the quality of the environment from this raw
data, based on their knowledge of the ecology of each species
and previous experience of finding them in samples from rivers of
a particular quality, the data is impenetrable to others, including
river managers and other customers of biological monitoring.

Biologists therefore convert their results into simple numerical
values that summarise the complex raw data so that others can
understand and interpret them.

A metric is any numerical parameter derived from biological
monitoring data to summarise it, such as totals (number of families,
number of individuals), diversity indices (see Chapter 5) and biotic
indices. Information is inevitably lost in these gross summaries,
so biologists still use the raw biological data for more detailed
analysis and interpretation.

As biologists working on rivers gained more knowledge about the
ecology and distribution of river invertebrates in relation to the
environmental pressures that they were concerned with (which
until the late 1980s was mainly water pollution from industry and
domestic sewage), more reliable biotic indices were developed
that were applicable across the whole country. The most
successful are biotic indices that relate the biological data to the
intensity of environmental pressures.

The Biological Monitoring Working Party score was the first index
that was used widely across the whole of the UK. (10)

The BMWP-score was developed for the 1980 National River
Quality Survey and the average BMWP score per taxon (BMWP
ASPT) and the number of BMWP-scoring taxa (BMWP NTaxa)
were used for status assessment until 2015.

BMWP is described in Chapter 5 Section 3.1.4. Although still
used in England for operational assessment in relation to water
resources, these indices have been replaced by a major revision
known as Walley Hawkes Paisley & Trigg indices. (41)

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