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CHAPTER 3 3.1 – 3.1.1 RIVPACS PREDICTIONS AND REFERENCE
3.1 The confounding influence of natural variation
3.1.1 The problem
Different natural invertebrate communities are found in This is why biotic indices cannot be used directly to assess
different types of stream, and biotic indices vary as much environmental quality across different rivers. They are fine
between different natural communities as they do because for comparing similar sites on the same watercourse – for
of pollution and other forms of damage caused by human example, comparing conditions upstream and downstream
activity. A poor index value could be caused by human of a discharge to assess its impact. See figure 3.6.
pressures or by natural conditions.
Figure 3.6
Results from the 1980 national river survey of England & Wales showing BMWP-scores from each monitoring site. There
is no way to distinguish low scores caused by harsh natural conditions from those caused by poor environmental quality.
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