Pesticides found in rivers and streams have been found to reduce the number of insects and other small aquatic life by as much as 42 percent.
A study of German, French and Australian rivers by scientists at the Helmholtz Association of German Research Centres found considerable losses in the regional biodiversity of aquatic insects and other freshwater invertebrates.
Their work included measurements of insecticides and fungicides, which are used often in agriculture and are typically well studied and heavily regulated when used on land.
However, the researchers said little examination has been done to gauge their effect on the streams and rivers they end up in after it rains and the chemicals are washed off farmland and into watercourses.
"The current practice of risk assessment is like driving blind on the motorway," said ecotoxicologist Matthias Liess co-author of the study.
"To date, the approval of pesticides has primarily been based on experimental work carried out in laboratories and artificial ecosystems.
"To be able to assess the ecological impact of such chemical substances properly, existing concepts need to be validated by investigations in real environments as soon as possible."
The study found that the damage to the biodiversity of rivers and streams is primarily due to the disappearance of several groups of species that are especially susceptible to pesticides.
These mainly include representatives of the stoneflies, mayflies, caddisflies, and dragonflies and are important members of the food chain right up to fish and birds.
The study concludes that the threat to biodiversity from pesticides has obviously been underestimated in the past.
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