Lakes and Watercourses

Assessment of current conditions
TABLE 25.
CURRENT CONDITIONS: total phytoplankton volume (mm 3 /l)
Class Description Biomass Biomass
May – October August
1 Very small biomass ≤ 0.5 ≤ 0.5
2 Small biomass 0.5 – 1.5 0.5 – 2.0
3 Moderate biomass 1.5 – 2.5 2.0 – 4.0
4 Large biomass 2.5 – 5.0 4.0 – 8.0
5 Very large biomass > 5.0 > 8.0
The seasonal mean of the phytoplankton biomass has been correlated to
a nutrient gradient where particular account has been taken of changes in
proportions between different groups of algae when deciding the
boundaries between the classes. Class boundaries for August values have
been set on the basis of an average correlation to the seasonal mean.
Class 1 represents oligotrophy, class 2 mesotrophy, class 3 and 4
eutrophy and class 5 hypertrophy. The algal biomass in ultra-oligotrophic
lakes (eg, large clearwater lakes and some mountain lakes) is usually
≤ 0.1 mm 3 /l, both as a seasonal mean and as a figure for August. Even a
slight input of nutrients to waters in this class causes marked structural
changes in the plankton community (including a manifold increase in
cyanobacteria).
TABLE 26.
CURRENT CONDITIONS: chlorophyll (µg/l)
Class Description Concentr. May – October Concentr. August
1 Low concentrations ≤ 2 ≤ 2.5
2 Moderate concentrations 2 – 5 2.5 – 10
3 High concentrations 5 – 12 10 – 20
4 Very high concentrations 12 – 25 20 – 40
5 Extremely high concentr. > 25 > 40
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