Planet WGSW-24010 User Manual page 97

be deemed significant, this being twice the mean difference to be expected from within-season
variation (6.1.3).
Other temporal changes in the MTR score which may arise, for example due to natural cycles of
plant growth, river conditions or river management works, are allowed for by use of the suffix
of confidence in the survey (4.5.7).
The MTR may also be influenced by the size of the river, its slope, substrate size, underlying
geology and the altitude of its source, as well as by nutrient status or other chemical
determinands. Allowance for this source of variation is made in the guidance provided on
selecting survey lengths and interpreting results: site comparability is one of the factors to be
taken into account when selecting survey lengths (Box 2) and survey lengths should only be
compared if they are physically similar (as expressed by the suffix of confidence, 4.6.12).
7.1.2 The need for quality assur ance
It is important that the sources of variation outlined above are reduced so that maximum possible
confidence can be placed in the accuracy, or 'quality' of survey results. This is normally
achieved by application of a quality control procedure, the aim of which is to minimise
unavoidable errors in carrying out the survey methodology; set quality targets and determine
whether these are being met; and provide a means for restoring quality if targets are not met.
Most standard quality control systems which are used for other biological surveys and chemical
analyses, such as control charts, ring sorts and resorting of samples, are not appropriate for
macrophytes. Macrophyte surveys produce data directly from a field survey. No samples are
taken, except for those required for identification or confirmation purposes. Re-surveys alone
are probably not sufficient, as they occur after the surveys have been undertaken and real changes
may have occurred in the interim. In addition, the de-limitation of the 'sample unit' available to
the re-surveyor (the upstream and downstream limits of the survey length and the definition of
the 'channel') is itself integral to the methodology and open to error.
The requirement for MTR surveys is therefore for a system of quality assurance, aimed at
minimising errors. Several aspects of MTR surveys can undergo quality assurance to a greater
or lesser degree. These include:
· operation of the method
· number of species
· abundance categories
· overall percentage cover
· identification
· database entry accuracy
In addition, relocation of survey lengths can be audited if required. This is recommended where
survey results at any one site are to be analysed from more than one visit (such as for UWWTD
monitoring) and the surveys undertaken by different a surveyor(s) on each occasion.
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