Planet WGSW-24010 User Manual page 93

which should be based on a site-by-site interpretation as specified above. To avoid mis-
interpretation of such maps, their use is discouraged.
6.2.4 Tempor al changes in tr ophic status
Although this application is untried, it is recommended that in the interim, the same essential
principles for interpretation of data be adopted as for spatial changes in trophic status. Thus, the
trophic status of a site should be determined by means of the standard descriptors provided in
Figure 4, and a change in MTR over time must be at least 4 units or 15% for it to be deemed
significant in terms of trophic status. This should mean that the change is greater than that which
may be expected from natural background variation and from sampler error. When assessing the
response of a river to nutrient-reduction measures, however, it is important to also interpret
temporal changes against the baseline variation for that river, as established over a number of
years prior to the commencement of nutrient reduction.
6.2.5 Futur e development
The limitations of the MTR, in terms of the complexity of interacting factors by which it may be
influenced, are recognised (Dawson et al 1999b). The guidance given above regarding
interpretation of data is thus the best available at the current time, given these limitations. Further
research and development to assess the effect of geology, altitude, flow type and other
environmental variables, is necessary before further refinement of the method can be made. This
may lead to the establishment of benchmark MTR values for river types which could be used to
assess the extent of deterioration from the typical value.
eutrophication impact may then be made.
R&D Technical Report E38
Nationwide comparisons of
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