Lakes and Watercourses

TABLE 34.
Reference values for planktonic algae in various lake types
Indicator Lowland lake, Lowland lake, Forest Mountain
shallow* deep* lake* lake*
Total volume, seasonal average, mm 3 /l 1 0.5 0.5 0.5
Total volume, August, mm 3 /l 1.5 0.75 0.5 0.5
Diatom biomass in April/May, mm 3 /l 1 1 0.5 **
Water-blooming cyanobacteria in August, mm 3 /l 0.5 0.5 0.05 **
No. of potentially toxin-producing 4 4 3 2
cyanobacteria (genera) in August
Gonyostomum semen, biomass in August, mm 3 /l 0.1 0.1 0.1 **
* Mountain lake: situated above the tree line
Forest lake: usually, but not always, situated above the reference elevation, referred to in Sweden
as ”the highest coastline”, where surrounding land comprises > 60% forest
Lowland lake: situated on mud sediments, usually below the highest coastline, where surrounding
land comprises > 60% cultivated land (arable land, meadow, fallow, other grazing land).
** No reference values available or the parameter is not relevant to the lake type.
Comments
Classification must be based on samples taken and analysed in accordance
with the Swedish EPA Environmental Monitoring Handbook.
Assessment need not necessarily be made using all parameters for a given
lake. The choice of parameters is governed by the aspects of the presence
of planktonic algae to be highlighted and the availability of background
data. Hence, in some cases assessment will be confined to harmful algae.
In others, assessment may focus on biomass quantities in parallel with the
presence of toxic algae.
In oligotrophic lakes, where there is often little difference from one
year to another, results from a single year may be used for assessment. In
moderately eutrophic and eutrophic lakes, on the other hand, data
should comprise figures from three years of study, so as to obtain mean
values. Variations between years due to weather have a particularly
marked impact on species balance and quantities of various algal groups
in such lakes, which in turn affects total biomass. If an assessment is made
using more limited data, this should be stated.
The scales for the respective parameters, as with the choice of parameters
and reference values, are intended to be used during a trial period.
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