Lakes and Watercourses

This classification is based on an overall assessment taking account of the
concentrations occurring in Swedish lakes at various degrees of human
impact.
Recorded concentration represents the mean value over three years
for the period May – October. By using 3-year mean values for August
alone, account must be taken of the element of uncertainty introduced.
Reference values can be calculated or estimated in a number of ways.
They can be estimated on the basis of historical studies of the area in
question or studies of similar but unaffected lakes in the vicinity. However,
phosphorus concentrations in some acidified lakes may be lower
than the original level. In the absence of other data, reference values can
be calculated using the correlation between total phosphorus and
coloured organic matter:
TP ref (µg P/l) = 5 + 48 · abs f 420/5
This function gives minimum observed values at a given absorbency and,
generally speaking, a higher degree of deviation. In some cases, the extent
of deviation may be estimated to be up to one class higher than in reality.
Clear mountain waters are a case in point. The function has been derived
from environmental monitoring programme data where series of at least
five years have been available. Taking account of the element of uncertainty
introduced, absorbency (abs f 420/5 ) can be calculated by multiplying
water colour (mg Pt/l) by 0.002.
In some limed or acidified lakes the ratio of recorded concentration to
reference value may be less than 1, which may indicate oligotrophication,
ie, a shift towards a more nutrient-poor state. Conditions of this kind
should be particularly noted so as to allow further study of possible
acidification-related effects.
TABLE 8.
DEVIATION from reference value, area-specific loss of total
phosphorus in watercourses
Class Description Recorded area-specific loss/reference value
1 No or insignificant deviation ≤ 1.5
2 Significant deviation 1.5 – 3
3 Large deviation 3 – 6
4 Very large deviation 6 – 12
5 Extreme deviation > 12
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