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The Directive also requires that the monitoring frequencies The Directive also points out that the times at which
take account of the variability of parameters resulting from monitoring is undertaken must minimise the impact of
natural and anthropogenic conditions. seasonal variation so that the results reflect changes only
as a result of anthropogenic pressure. Additional monitoring
Ultimately, the frequencies should allow for an acceptable during different seasons may be necessary to achieve this
level of confidence and precision, which must be stated in objective. However, samples for the same element must
the River Basin Management Plan. always be collected in the same seasons to avoid natural
seasonal effects from influencing the monitoring results.
Low confidence and precision can cause changes of class
that are not easily explained, as well as making it more In the temperate northern hemisphere climates of Europe
difficult to justify more expensive programmes of measures and the UK, the normal default frequencies are presented in
that may be necessary to restore quality to meet the Table 1.4 below.
environmental objectives.

Table 1.4
UK Normal Monitoring Frequencies for WFD

Parameter Frequency
Macroinvertebrates Twice in the appropriate year (Mar – May and Sept – Nov)

Macroinvertebrates
(acidification Wales only) Once in the appropriate year (Mar – May)

Macrophytes Once in the appropriate year (June – Sept)

Diatoms Twice in the appropriate year (Mar – May and Sept – Nov)

Once in the appropriate year (April – Sept) for surveillance monitoring (should be
River Habitat Survey
performed once every 6 years)

Similar frequencies should be used for operational
monitoring because of the similar requirements for
status classifications based on this.
Unless it is to be used for status classification,
investigative monitoring is not subject to the
same restrictions. Most investigative surveys are
not repeated. However, seasonal effects must be
taken into consideration and may dictate when
investigative monitoring is undertaken.

Sometimes, investigative surveys have to be
undertaken at times of the year that are not ideal:
for example, surveys to investigate the impacts
of pollution incidents. Most models for predicting
reference conditions assume that samples are
collected in particular seasons, so it may not be
possible to use samples collected outside these
seasons to estimate status.

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