Lakes and Watercourses

TABLE 13.
CURRENT CONDITIONS: turbidity
Class Description FNU units
1 No or insignificant turbidity ≤ 0.5
2 Slightly turbid 0.5 – 1.0
3 Moderately turbid 1.0 – 2.5
4 Substantially turbid 2.5 – 7.0
5 Highly turbid > 7.0
In lakes, turbidity is classified using seasonal mean values (May –
October) over one year, based on monthly readings taken in surface
water (0.5 m) or in samples taken at several depths. Assessment of
watercourses should be based on samples taken 12 times over one year.
The assessment scale classifies group concentration levels typical of
Swedish lakes and watercourses and is not related to biological or
microbial effects.
Turbidity readings using different methods yield somewhat different
results. The previous Swedish standard expressed turbidity as FTU
(formazine turbidity units). The present Swedish and ISO standard states
readings in the form of FNU (formazine nephelometric units). Other
methods involve readings expressed as NTU (nephelometric turbidity
units) or JTU (Jackson turbidity units). For practical purposes, 1 FTU = 1
FNU = 1 NTU ≈ JTU.
TABLE 14.
CURRENT CONDITIONS: Secchi depth in lakes
Class Description Depth (m)
1 Very great Secchi depth ≥ 8
2 Great Secchi depth 5 – 8
3 Moderate Secchi depth 2.5 – 5
4 Little Secchi depth 1 – 2.5
5 Very little Secchi depth < 1
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