Planet WGSW-24010 User Manual page 95

Figur e 6 Relationship between the number of scor ing species and the phosphate
concentr ation. (Figure 39, R&D Technical Report E39, Dawson et al 1999b).
6.4 Inter pr etation of over all per centage cover values
Overall percentage cover is, by itself, of little use in terms of assessing trophic status. There is
no relationship between overall percentage cover and phosphate concentration. It is still a useful
parameter to record, however, for the following three reasons.
· In many cases it acts as a double check on the percentage covers given to individual scoring
species.
· In some cases, eutrophication may cause a change in overall percentage cover, and if so is
worthy of note.
· Excessive growth of macrophytes, resulting in very high overall percentage cover values, can
in itself result in problems to users of rivers, giving rise to complaints, and may impact on
river management functions. Where such complaints or impacts arise, it is important to
establish whether the excessive growth is due to eutrophication, or to other factors. Where
it is due to eutrophication, then this clearly constitutes an 'undesirable disturbance' in
UWWTD terms.
Where a marked change is recorded, determine whether this is a result of a change in trophic
status by establishing to which key species the change in overall cover and the change in MTR
can be attributed. If the key species is/are the same, then the change in overall cover is likely to
be due to a change in trophic status. If so, then this is worthy of mention when reporting results.
Where the change is a very marked increase in overall percentage cover, such that it may give
rise to complaints from river users (either on aesthetic grounds or as a result of physical
disturbance to use of the river), then this clearly constitutes an 'undesirable disturbance' in terms
of the definition of eutrophication given in the UWWTD.
Perhaps the most notable example of this relates to situations where the key species is
Cladophora. If an increase in overall percentage cover and a decrease in MTR is due mainly to
an increase in percentage cover of Cladophora, then it may be deduced that the increase in
overall percentage cover is a manifestation of eutrophication.
R&D Technical Report E38 81