Report from the Sub-comittee on the environment and health

Effects on primary
producers
The effect of pesticides depends on whe<strong>the</strong>r <strong>the</strong> recipient is a
watercourse or stagnant water. The effect of pesticides on aquatic
organisms generally depends on <strong>the</strong> retention time and thus on <strong>the</strong>
exposure time. In watercourses, <strong>the</strong> retention time depends on <strong>the</strong>
average rate of flow and <strong>the</strong> presence of areas with a low rate of flow. In
a watercourse to which a herbicide was added, a stretch with a high rate
of flow directly downstream <strong>from</strong> <strong>the</strong> treated area was analysed and
compared with a more slow-flowing stretch 225 m downstream
(Thomson et al. 1995). Although <strong>the</strong> concentration was briefly higher in
<strong>the</strong> upstream stretch in connection with <strong>the</strong> spraying, <strong>the</strong> retention time
of <strong>the</strong> substance in a concentration of more than 1 microgramme per litre
was twice as long in <strong>the</strong> stretch of water 225 m downstream. In
watercourses, <strong>the</strong> concentration of pesticides also decreases at a given
distance downstream of <strong>the</strong> source, depending on dilution and
deposition/metabolism. In stagnant water, <strong>the</strong> effect of <strong>the</strong> pesticides
depends particularly on <strong>the</strong> volume of water and <strong>the</strong> rate of water
replacement.
Small and slow-flowing watercourses and small ponds are <strong>the</strong>refore most
exposed to any pesticides. Moreover, <strong>the</strong>se systems are closely linked to
<strong>the</strong> surroundings and are <strong>the</strong>refore more likely to be affected by any use
of pesticides in <strong>the</strong> surrounding areas.
Biologically, watercourses differ <strong>from</strong> stagnant waters by being open
systems. There is permanent drift of organisms with <strong>the</strong> current, which
means that stretches of watercourses are generally recolonised quickly.
That means that watercourses are generally very resilient and <strong>the</strong>refore
quickly return to normal once a chemical impact has ended.
There are many documented instances of effects of herbicides on primary
producers in freshwater ecosystems, whereas insecticides seldom seem to
have any direct effect on plants. The effect of herbicides is usually that
<strong>the</strong>y reduce productivity, whereas, in <strong>the</strong> short term, <strong>the</strong> biomass is not
affected. There is a clear tendency for <strong>the</strong> adverse effects to end very
quickly when <strong>the</strong> exposure is over. Besides <strong>the</strong> direct effects of
herbicides on primary producers in watercourses, indirect effects have
been found in a number of studies. For instance, photosyn<strong>the</strong>sis in an
algae community increased at a concentration of more than 4
microgrammes per litre of <strong>the</strong> insecticide lindane owing to a reduction in
<strong>the</strong> number of grazing invertebrates (Pearson, Crossland 1996).
With <strong>the</strong> concentrations found for <strong>the</strong> herbicide atrazine, it is unlikely
that algae and macrophytes will be affected. Generally speaking, <strong>the</strong>
concentration has to be much higher than 50 microgrammes per litre
before <strong>the</strong> ecosystems are affected (Solomon et al. 1996). In <strong>the</strong> case of
<strong>the</strong> herbicide hexazinone, <strong>the</strong> measured concentrations are also so low
that no serious effect on <strong>the</strong> primary producers is expected. However, up
to 43 microgrammes per litre have been found in a drain water sample,
which could affect aquatic plants locally if <strong>the</strong>re were little dilution in <strong>the</strong>
recipient watercourse. EC50 (4 hours) for hexazinone has been found to
be 3.6 microgrammes per litre for aquatic plants in watercourses, and a
concentration of 145-432 microgrammes per litre reduced <strong>the</strong>
productivity of aquatic plants by 80% (Schneider et al. 1995).
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