Page 145 - Freshwater-Biology-and-Ecology-Handbook

Because Chironomidae and Oligochaeta are found in almost could be allocated to each family far more accurately than
all invertebrate samples, the index values derived for these they could to species because a far larger data set was
taxa were essentially the average value across the whole available from which to derive the index values. We did not
quality spectrum. That would have elevated WHPT ASPT base WHPT indices on families to save costs.
values at sites with strong organic pollution, when these taxa
but few others are present. As a result, the definitive index The regulatory agencies in Great Britain actually analyse
values used for Oligochaeta were those derived for the invertebrate samples to a mixed taxonomic level that is
most tolerant family, Tubificidae, and index values used for mostly to species, so that monitoring data can be used
Chironomidae were the values for Chironomini, as they had to track invasive species as well as subtle effects from
been in the BMWP index to allow the most polluted sites to emerging pressures such as climate change. Because they
have very low WHPT ASPTs. share similar physiology, many species of river invertebrates
belonging to the same family have similar tolerances to
We considered developing species-level indices to increase pollution and other pressures, with only a few notable
the number of taxa from which the index is calculated and exceptions. This causes species-level versions of indices
to narrow the range of conditions from which each taxon is that also have family-level versions to offer little additional
found. When WHPT was developed, however, index values precision or accuracy.

Like BMWP, WHPT is used in two WHPT NTaxa is a measure of taxonomic richness
complementary forms: the average WHPT and it (and status classification based on it) responds
score per taxon (WHPT ASPT) and the number to all types of environmental pressures, including
of WHPT scoring taxa (WHPT NTaxa). toxic chemicals.
We use this index because WHPT ASPT cannot
WHPT ASPT is the average sensitivity of detect certain pollutants, in particular those caused
invertebrates to organic and nutrient loads and other by acidification and metal pollution, which is common
pressures associated with domestic or mixed waste in mine water and industrial discharges. WHPT NTaxa
discharges (fine sediment, ammonia toxicity) or that is less precise than WHPT ASPT because it is not an
affect dissolved oxygen and productivity (reduced average, and it is more sensitive to laboratory error
flow, reduced shading). and sampling variation. Beware that poorer NTaxa
(ie fewer taxa) sometimes represents better quality:
Status classification based on WHPT ASPT reflects in oligotrophic upland streams, particularly with low
these pressures, which are still the most widespread alkalinity, mild organic pollution can increase NTaxa,
combination of environmental pressures that impact although the composition of taxa may change and be
river invertebrate communities. Although nominally reflected in poorer ASPT.
an index of saprobity (organic loading), WHPT ASPT
is actually a multi-pressure index (as are all indices of The combination of WHPT-ASPT and WHPT NTaxa
organic pollution) because all these pressures interact responds to virtually all combinations of pressures.
and can co-exist. River invertebrates are particularly Using WHPT ASPT and WHPT NTaxa together not
sensitive to dissolved oxygen and anything that only provides a measure of ecological quality but
impedes their respiration. also gives a limited diagnostic capability that enables
sites affected predominantly by organic and related
The wide range of tolerances of different taxa enables pressures to be differentiated from those affected by
the strength of these pressures to be measured toxic pressures. High WHPT ASPT but low WHPT
relatively accurately across a wide range of intensity. NTaxa indicates an absence of organic or related
The precision of WHPT ASPT is enhanced by being an pressures (indicated by the presence of sensitive taxa)
average and is relatively insensitive to sampling effort. but the presence of another type of pressure, which
must either be toxic metal/acidification or habitat
degradation. When both WHPT ASPT and WHPT
NTaxa are calculated, the WHPT score is redundant.

Score = ASPT x NTaxa

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