Pesticides not only affect organisms in the fields, they also find their way into the surrounding aquatic ecosystems. A study has now revealed that small bodies of water in German agricultural landscapes are much more polluted by pesticides than previously thought. In addition, supposedly tolerable values also have a significant impact on the biodiversity of the water dwellers.
The pesticides used in conventional agriculture secure crop yields by fighting harmful insects, fungi and weeds. Nevertheless, they have long been controversial: some are considered to be potentially carcinogenic and are suspected of causing the decline in insects and poisoning birds. In addition, the pesticides also find their way into the surrounding waters and damage the aquatic ecosystems, which are part of the food web and support the self-purification of the water and play a role in the preservation of biodiversity.
Pesticide levels over the limit
But to what extent are small bodies of water in German agricultural landscapes already contaminated by pesticides? A research team led by Matthias Liess from the Helmholtz Center for Environmental Research investigated this in a nationwide monitoring. For two years, the scientists tested the pesticide load at more than 100 measuring points on streams that flow through mainly agricultural lowland regions in twelve federal states. Liess and his colleagues also wanted to find out how much the aquatic biodiversity is affected by such inputs.
The result: “We have demonstrated a significantly higher pesticide load in the small bodies of water nationwide than we originally expected,” said Liess. The pollution was so high in over 80 percent of the waters that it exceeded the so-called RAK values. These values denote the concentration of an active ingredient specified in the official approval procedure for a plant protection product, which must not be exceeded in the water in order to prevent negative effects on aquatic organisms. In 18 percent of the streams, more than ten pesticides were found to be exceeded. For example, thiacloprid, an insecticide from the class of highly potent neonicotinoids, exceeded the RAK value in three bodies of water by more than 100 times. Other insecticides such as clothianidin, methiocarb and fipronil, but also herbicides such as terbuthylazine, nicosulfuron and lenacil exceeded the RAK value in over 25 bodies of water by a factor of ten to 100.
Current limit values set too high
Using their data, Liess and his team were also able to prove that the current RAK values are set too high to protect the ecosystems and their biodiversity. According to their data, the pesticides already act on communities of aquatic invertebrates in much lower concentrations than previously assumed in the pesticide approval. The researchers were able to determine that there are only a reduced number of endangered species in most of the agricultural streams.
The concentration at which the pesticides act on the communities depends on which species are to survive: If, for example, sensitive insect species such as caddis flies (Trichoptera) and dragonflies are to be protected in small bodies of water, the limit values required are around 1,000 times lower than if insensitive snails and snails Worms should be preserved. “For sensitive insect species, the pesticide concentration in the small lowland waters is the essential factor that determines their survival,” explains Liess. “Other environmental problems such as water development, lack of oxygen or too high a nutrient content, on the other hand, play a lesser role.”
New measurement methods required
According to the researchers, the fact that the current licensing practice of pesticides underestimates the high sensitivity of species in the ecosystem is also due to the fact that the assessment is based heavily on laboratory studies and models. “In addition to pesticides, numerous other stressors act on the organisms in the ecosystem, making them much more sensitive to pesticides,” explains Liess. “Natural stressors such as the pressure of predators or the competition between species are not sufficiently taken into account in the approval process.”
According to the research team, the type of sampling is also often not realistic: Instead of taking a scoop sample specified as a standard by the EU Water Framework Directive, the researchers recommend the event samples they use. An automatically controlled sampler takes the water samples after a precipitation event. “The event sample provides much more realistic results, as the pesticides are carried into the water, especially when there is rain, as a result of surface runoff from the field,” says Liess. As a result, these samples are ten times more stressed than the scoop samples.
Overall, Liess and his colleagues therefore recommend evaluating water pollution in the future through regular official environmental monitoring with event samples. In addition, new findings should be incorporated into the approval process for new plant protection products more quickly than before, according to the team. “The fact that pesticides are still used today that were approved many years ago and are therefore often based on an outdated risk assessment must change as quickly as possible,” says Liess. “Only in this way can we preserve the biodiversity in our waters and with it the services that these communities provide for our ecosystems.”
Source: Helmholtz Center for Environmental Research, Article: Water Research, doi: 10.1016 / j.watres.2021.117262