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The Water Framework Directive (WFD),
requires European Member States to assess the
‘‘ecological status’’ of surface waters. As part of this,
many European countries have developed an ecological
quality classification scheme for chlorophyll
concentrations as a measure of phytoplankton abundance.
The assessment of ecological quality must be
based on the degree of divergence of a water body
from an appropriate baseline, or ‘reference condition’.
It is, therefore, necessary to determine chlorophyll
reference conditions for all European lake types. This
involves examining how chlorophyll concentrations
vary by lake type, in the absence of any nutrient
pressures from agriculture or wastewater. For this
purpose, a dataset of 540 European lakes considered
to be in a relatively undisturbed reference condition
has been assembled, including data on chlorophyll
concentration, altitude, mean depth, alkalinity, humic
content, surface area and geographical region. Chlorophyll
was found to vary with lake type and
geographical region, and to be naturally highest in
low-altitude, very shallow, high alkalinity and humic
lake types and naturally lowest in clear, deep, low
alkalinity lakes. The results suggest that light and
mineral availability are important drivers of chlorophyll
concentrations in undisturbed lakes. Descriptive
statistics (median and percentiles) of chlorophyll
concentrations were calculated from populations of
lakes in this reference lake dataset and used to derive
lake-type specific reference chlorophyll concentrations.
These reference conditions can be applied,
through a comparison with observed chlorophyll
concentrations at a site, in the assessments of ecological
status and provide a consistent baseline to adopt
for European countries.
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