Planet WGSW-24010 User Manual page 84

2. Interpret each set of results in terms both of trophic status and of the significance of
impact from the QD, by use of a standard set of 'rules', a set of standard descriptors being
used to describe the resulting conclusions. The 'rules' and descriptors are given below
in Section 6.1.3.
3. Assess the consistency of results in the same reach on different sampling occasions to
verify that any eutrophication impact observed is ongoing and consistent, and that
anomalous data are not due to anomalous weather conditions or conditions of atypical
flow. Do not, however, compare individual scores between different years.
6.1.3 Standar d Appr oach to Inter pr etation of MTR Results
Under lying Pr inciples
This approach is based upon a standard set of decisions with the following underlying principles:
i) Sites with MTR scores of more than 65 are unlikely to be eutrophic. However, these sites
could be at risk of becoming eutrophic and the MTR should be compared with that
expected in an un-impacted, physically similar reach.
ii) Sites with MTR scores of less than 25 are badly damaged by either eutrophication,
organic pollution, toxicity or are physically damaged.
iii) Sites with MTR scores between 25 and 65 are likely to be either eutrophic or at risk of
becoming eutrophic. However, as the MTR may be limited purely by the physical nature
of the site, the MTR should be compared with that expected in an un-impacted, physically
similar reach. This is probably most relevant to those sites with MTRs between 45 and
65 (see Table 1); below an MTR of 45, it is likely that the site is impacted by
eutrophication. Sites with a high number of species which are unimpacted may often
have MTR scores in the range 45–65. This is due to the large number of species with
STRs of 4–6 which biases the MTR to 40–60. Sites which are obviously unimpacted (eg
>20 species present) should be recognised as such within this category.
iv) A downstream change in MTR is deemed to be significant if the difference is at least 4
MTR units or 15%.
A predictive framework of the MTRs to be expected in un-impacted rivers of different physical
types, although currently in the early stages of development, is currently not yet available. To
assist operational decisions until such a framework is available, reference may be made to two
sources of information.
Firstly, broad comparisons can be made using MTR data from other local sites with similar
altitude, geology and flow characteristics but lower nutrient concentrations. The importance of
undertaking more comprehensive catchment surveys in this respect is emphasised in 2.4.3 and
such sampling strategies are to be encouraged as good practice.
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