Lakes and Watercourses

concentrations. Using accepted terminology, the classes correspond to
oligotrophy (1), mesotrophy (2), eutrophy (3 and 4) and hypertrophy (5).
There is good reason to define a further characteristic sub-group with
concentrations below 6 µg/l within the oligotrophic range. This group
represents ultra-oligotrophy.
Total nitrogen is more variable during the season than total phospho-
TABLE 3.
CURRENT CONDITIONS: concentration of total nitrogen
in lakes (µg/l)
Class Description Concentration May – October
1 Low concentrations ≤ 300
2 Moderately high concentrations 300 – 625
3 High concentrations 625 – 1.250
4 Very high concentrations 1.250 – 5.000
5 Extremely high concentrations > 5.000
rus and is therefore unsuitable for assessments based on concentrations in
August. Inorganic nitrogen (nitrate + ammonium) peaks markedly in late
winter and organic nitrogen reaches a peak in the summer. It is therefore
difficult to describe a general pattern of variation for total nitrogen.
TABLE 4.
CURRENT CONDITIONS: ratio of total nitrogen/
total phosphorus in lakes
Class Description Ratio June – September
1 Nitrogen surplus ≥ 30
2 Nitrogen – phosphorus balance 15 – 30
3 Moderate nitrogen deficit 10 – 15
4 Large nitrogen deficit 5 – 10
5 Extreme nitrogen deficit < 5
The assessment scale is intended to group concentrations that are
typical of Swedish lakes and is not related to biological/microbial effects.
The scale refers to the mean value during June – September over one
year based on monthly readings taken in the epilimnion or, if only one
sample is taken, surface water (0.5 m). The ratios are weight-based and
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