Lakes and Watercourses

16
The current conditions scale in this report is based on the levels occurring
in Sweden for each parameter and, to some extent, on the biological
and other conditions characterising different levels. Hence, as far as
possible, the boundaries between the classes coincide with clear changes
on the relevant gradient. Where it has not been possible to identify levels
where such changes occur, boundaries have been decided statistically on
the basis of the most representative data possible, or arbitrarily on the
basis of an overall judgement of what can be considered reasonable.
The width of the classes varies, depending on the way each parameter
changes along a gradient from low to high or vice versa. In some cases,
changes in the lower range of the scale are particularly significant. Here
scales have been graduated to give them sufficient resolution in this
respect. In other cases, changes occur gradually along a gradient and the
boundaries between classes are more evenly distributed along the scale.
The way scales have been developed for individual parameters may be
seen in each chapter.
It should be emphasised that the assessment scales cannot easily be
interpreted as representing ”good” or ”bad” environmental quality. The
parameters must be evaluated individually in the light of the quality
aspects they are intended to reflect. This applies particularly to the scales
for assessment of current conditions. A number of examples serve to
illustrate this. Chemical parameters such as metals, organic pollutants
and alkalinity are fairly closely related to water quality in the sense that
increasing concentrations (decreasing for alkalinity) can generally be said
to reflect a growing risk of negative effects on aquatic organisms or use of
water resources. The scale for total phosphorus reflects conditions for
increasing quantities of phytoplankton in water. From an aesthetic viewpoint,
for bathing, water supply etc, an increase of this kind is generally
considered undesirable, but in production terms, the scale should have
been placed the other way round. This has not been done because the
total phosphorus parameter is primarily intended to indicate conditions
for the presence of phytoplankton and associated adverse effects. This
rationale also underlies the form chosen for assessment scales for other
parameters. Thus, classification of the state of fish assemblages is based
on expected fish numbers and diversity of fish species. Class 1 (lakes)
indicates a large number of fish, a large number of species with high
diversity and a high proportion of piscivorous species, ie, a rich and
diverse fish community. Class 1 watercourses feature a large number of
salmonids with high breeding success. Class 3 indicates that the fish
assemblages in the lake or watercourse are average for Swedish waters,
whereas class 5 indicates assemblages poor in numbers of species and
individuals. In the case of parameters where contradictory interpretations
or evaluations are possible, the direction of the scales has been decided