Isoetids, as indicators of near-pristine softwater
lakes, have a high priority in national and international
(European Water Directive Framework) assessments of
ecological lake quality. Our main goal was to identify the
most important environmental factors that influence the
composition of plant communities and specifically determine
the presence and abundance of the isoetid Lobelia
dortmanna in NW European softwater lakes. Geographical
position and composition of surface water, porewater,
sediment and plant communities were examined in 39 lakes
in four regions (The Netherlands, Denmark, West Norway
and East Norway) distributed over a 1,200-km long distance.
We confirmed that lake location was accompanied
by significant changes in environmental variables between
NW European lakes. Lake location was the single most
important determinant of vegetation composition and it had
significant individual contributions independent of the
coupling to environmental variables. This influence of
location was supported by a significant decline of community
similarity with geographical distance between pairs
of lakes at regional, inter-regional and international scales.
Combining the geographical position with environmental
variables for surface water, porewater and sediment significantly
improved prediction of vegetation composition.
Specifically, the combination of latitude, surface water
alkalinity, porewater phosphate and redox potential offered
the highest correlation (BIO ENV correlation 0.66) to
vegetation composition. This complex analysis can also
account for high sediment variability in the littoral zone of
individual lakes, by using site-specific physico-chemical
sediment factors, and offer better predictions of vegetation
composition when lake water chemistry is relatively
homogeneous among lakes within regions