Report from the Sub-comittee on the environment and health

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ensure drainage of surrounding fields, whereby <strong>the</strong>y have been made
unnaturally wide and physically uniform. Both <strong>the</strong> degradation time and
<strong>the</strong> effect on <strong>the</strong> ecological conditions presumably depend on <strong>the</strong>
physical heterogeneity, so – all else being equal – physically uniform
watercourses will be affected most by any supply of pesticides.
5.3.1 Conclusions
There are very few Danish studies of <strong>the</strong> occurrence and effects of
pesticides in Danish watercourses and ponds and, as yet, no systematic
studies in lakes and coastal waters. The assessment of <strong>the</strong> effects of
pesticides has <strong>the</strong>refore been based primarily on foreign studies, and
importance has been attached to field studies. It has not been possible,
ei<strong>the</strong>r, to find any studies of <strong>the</strong> effects of all <strong>the</strong> pesticides detected in
Danish freshwater ecosystems. Since <strong>the</strong>re are often differences in <strong>the</strong>
species composition and <strong>the</strong> structure and function of <strong>the</strong> ecosystems in
Denmark, compared with <strong>the</strong> foreign studies, <strong>the</strong> sensitivity to a given
pesticide should be treated with caution. Owing to <strong>the</strong> relatively low
water temperatures in Denmark, compared with more sou<strong>the</strong>rly
countries, <strong>the</strong> degradation time for pesticides is shorter in Denmark, so
<strong>the</strong> exposure time and thus <strong>the</strong> risk of effects are greater. With our
present level of knowledge it is <strong>the</strong>refore difficult to judge how <strong>the</strong>
present use of pesticides affects Denmark’s freshwater systems.
However, several measurements indicate that pyrethroids and
thiophosphate insecticides have been detected in concentrations at <strong>the</strong>
level that has effects according to <strong>the</strong> existing literature. In particular, <strong>the</strong>
available concentration levels indicate that it is insecticides – and
especially <strong>the</strong> pyrethroids – that can have an adverse effect. In addition,
by reason of <strong>the</strong>ir persistence, <strong>the</strong> pyrethroids can occur in freshwater
ecosystems for a long period, during which <strong>the</strong>y can be absorbed by, for
example, invertebrates that live <strong>from</strong> dead organic material.
In connection with authorisation of pesticides, tests are carried out to
determine <strong>the</strong>ir toxic effect on individual species of algae, daphnia, fish
and o<strong>the</strong>r organisms. Mesocosm analyses that simulate entire ecosystems
are also performed. However, <strong>the</strong>se analyses do not provide insight into
<strong>the</strong> many natural factors that interact in nature and <strong>the</strong>y do not cover <strong>the</strong>
combination of <strong>the</strong> many different pesticides detected in <strong>the</strong> aquatic
environment.
It is <strong>the</strong>refore likely that <strong>the</strong> freshwater environment is affected by <strong>the</strong>
present use of pesticides, but it is not yet possible to determine <strong>the</strong>
magnitude of <strong>the</strong> impact owing to lack of data and knowledge. The
county authorities, in <strong>the</strong>ir regulatory capacity, have provisionally
estimated that around 2% of <strong>the</strong> unfulfilled targets in approx. 11,000 km
of watercourses are due to toxic substances, including pesticides
(Windolf 1997). However, this figure is based on a subjective evaluation
and will also vary with <strong>the</strong> region, <strong>the</strong> sampling method and <strong>the</strong>
frequency.
The following specific conclusions can be drawn:
• Some pesticides – particularly pyrethroid insecticides and
thiophosphates – are toxic to aquatic organisms in lower