Logistics Of Sampling - Planet WGSW-24010 User Manual

3.3

Logistics of sampling

3.3.1 Timing of sur veys
Macrophyte surveys should be carried out between mid-June and mid-September (the 'survey
season') after several days of low flow or low–normal flow as opposed to high flow/spate.
Although some macrophytes are visible outside this survey season, others are not and hence the
MTR score will not be an accurate representation of the trophic status because of the missing
species. Even within the survey season, differences in the growth patterns of individual species
may result in changes in their relative abundance during the season, with consequent variation
in the MTR score. To minimise such within-season differences in MTR score, surveys should
be undertaken in close succession when comparing different sites on the same river within the
same year, and at the same time of the survey season each year when comparing the same site in
different years.
Rivers should not be surveyed at times of high flow as access is dangerous and turbid conditions
mean complete and/or accurate data are unlikely to be obtained. Once spate water levels have
dropped to, and remained at, more normal flow levels for several days, surveys can be resumed
but the survey results may be affected and this will need to be taken into account when
interpreting the data. The timing of the spate flow will determine which macrophyte species are
able to grow back to their original abundance levels and which will be under represented for the
remainder of the season.
Where surveys being compared cannot be undertaken at the same time of the survey season,
caution should be applied when comparing results and allowance made for natural within-season
variation. Macrophyte species have different seasonal growth patterns, some species exhibiting
accelerated growth rates early in the season and others not attaining maximum size until late
summer. This can result in changes in the MTR and the overall percentage cover from early to
late summer.
3.3.2 Number of sur veys per year
To assess a site for trophic status, it is recommended that a minimum of one survey per year for
three years should be undertaken, with the surveys being carried out at the same time of the
season each year.
A useful way to organise a survey programme to ensure that surveys are undertaken at the same
time of the season each year, is to survey sites in the same order each year.
If a second survey is undertaken at a site within the same year, then there should be a minimum
of seven weeks between surveys and allowance for within-season differences in growth must be
made when interpreting results. It is recommended that such surveys are undertaken only for
calibration purposes, to provide an indication of the within-season variation and to help when
placing overall results into a wider context.
R&D Technical Report E38 22