Lakes and Watercourses

used to estimate the distribution of benthic vegetation, for example, since
double the Secchi depth is regarded as a rough measure of compensation
depth, ie, the depth at which photosynthesis does not occur.
Only current conditions scales are given for the above parameters.
Assessment of deviation from reference values is hindered by the absence
of background data more than is the case with other quality parameters.
Assessment of current conditions
TABLE 12.
CURRENT CONDITIONS: water colour
Class Description Abs f 400/5
Colour figure
(mgPt/l)
1 Clear, or hardly discoloured water ≤ 0.02 ≤ 10
2 Slightly discoloured water 0.02 – 0.05 10 – 25
3 Moderately discoloured water 0.05 – 0.12 25 – 60
4 Substantially discoloured water 0.12 – 0.2 60 – 100
5 Heavily discoloured water > 0.2 > 100
In lakes, classification of water colour is based on seasonal mean values
(May – October) over one year, based on monthly readings taken in
surface water (0.5 m) or in samples taken from several depths.
Assessment of watercourses should be based on samples taken 12 times
over one year. The assessment scale is intended to group water colour
levels typical of Swedish lakes and watercourses and is not related to
biological or microbial effects.
Photometer readings of the absorbency of filtered water (0.45 µm
membrane filter) in a 5 cm cuvette at a wavelength of 420 nm give
greater accuracy than readings of water colour using a colour comparator,
particular at low colour levels. Photometer readings are therefore
preferable. The table shows a multiplication factor of 500, used to
convert absorbency units (abs f 420/5 ) to a colour value. This factor
involves an element of uncertainty.
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