Page 68 - Freshwater-Biology-and-Ecology-Handbook

CHAPTER 1 5.7 – 5.7.1 5.7 Reference conditions

The main goal of stream assessment, according
to the Water Framework Directive, is to classify
water bodies into status classes (high, good,
moderate, poor or bad) and these are defined
by their deviation from type-specific reference
conditions. The basic principle of biological
quality assessment in the WFD is to compare
the actual biological communities in the water
body with the biological communities that
should be there if it was in a natural state –
specifically, the actual values of biological metrics
with their values at reference conditions of the
relevant water type.

The reference condition is, by definition, ‘high status’. The
general definition of high status in the WFD is: There are no,
or only very minor, anthropogenic alterations to the values
of the physico-chemical and hydromorphological quality
elements for the surface water body type from those normally
associated with that type under undisturbed conditions. The
values of the biological quality elements for the surface water
body reflect those normally associated with that type under
undisturbed conditions, and show no, or only very minor,
evidence of distortion. These are the type-specific conditions
and communities. (Table 1.2, Annex V, WFD).

Reference conditions are described in more detail in
CIS Guidance No 10 Rivers & Lakes – typology, reference
conditions and classification systems. The definitions of
reference were refined further during intercalibration and
are described later in this section.
Ideally, reference values are derived from reference
sites that have no (or minimal) alterations as a result of
human interference or pressure. UK TAG recommends
that: reference conditions should reflect a state in the
present or in the past corresponding to very low pressure,
without the effects of major industrialisation, urbanization
and intensification of agriculture, and with only very minor
modification of physico-chemistry, hydromorphology and
biology.

This is not always possible, so reference values
may be determined using:

• networks of reference sites
• modelling approaches
• or, where the above are not possible (even in
combination), expert judgement.

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