Lakes and Watercourses

winter/spring, i.e. ice-covered period, late summer/autumn) over three
years are assessed for all lakes.
Annual minimum values for watercourses are also assessed, although
here assessment should be based on samples taken 12 times a year over
three years. It may be necessary to monitor concentrations more
frequently in small watercourses, particularly during the summer.
TABLE 10.
CURRENT CONDITIONS: oxygen concentration (mg O 2 /l)
Class Description Annual minimum concentration
1 Oxygen-rich ≥ 7
2 Moderately oxygen-rich 5 – 7
3 Moderately oxygen-deficient 3 – 5
4 Oxygen-deficient 1 – 3
5 No or almost no oxygen ≤ 1
Note: The presence of hydrogen sulphide (H 2 S) is indicated by ††
Samples from the deepest point in a stratified lake sometimes give a
misleading picture of oxygen state if only a very small proportion of the
total volume of the lake is deep water. To avoid this, a rule of thumb
should be that readings taken from localities or sampling depths representing
at least 10 per cent of the bottom area of the lake should be used
to reflect the oxygen status of stratified lakes.
TABLE 11.
CURRENT CONDITIONS: organic matter
(oxygen-consuming substances)
Class Description Concentration as TOC or COD Mn (mg/l)
1 Very low concentration ≤ 4
2 Low concentration 4 – 8
3 Moderately high concentration 8 – 12
4 High concentration 12 – 16
5 Very high concentration > 16
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