Ducks, stoneflies and tubeworms are among the diverse group of invertebrates that spend much of their lives as larvae in water. These and many other invertebrates play a crucial role in freshwater ecosystems. They are essential for the decomposition of organic matter, water filtration and nutrient exchange between water and land. "Invertebrates have long been the cornerstone of water quality monitoring," explains the study's lead author, Prof. Dr. Peter Haase of the Senckenberg Research Institute and Natural History Museum in Frankfurt. "Such monitoring is extremely important because rivers and lakes are under great pressure from human activities. They are among the ecosystems most threatened by biodiversity loss".

Recovery impeded

Inland waters are under pressure from a variety of pressures from agricultural and urban land use. Pollutants, organically polluted sewage, fine sediments and pesticides accumulate in the water. In the 1950s and 1960s, water quality in many rivers was very poor due to high levels of pollution, for example from untreated sewage discharges. This led to the disappearance of many freshwater species. Improvements in water quality from the 1980s onwards, particularly the construction of sewage treatment plants, led to significant improvements.

The introduction of the Water Framework Directive in 2000 led to the implementation of restoration measures to return rivers to good ecological status, which helped recovery. "But the recovery was less in urban and agricultural areas and where there were dams," says Ralf Verdonschot, co-author from Wageningen University & Research. "The increase in biodiversity stopped around 2010. Continuing pressures, such as too many nutrients and water abstraction, and new pressures, such as new chemical pollutants and more climate extremes, hindered further recovery."

Better wastewater management needed

The team analysed a large dataset collected between 1968 and 2020 in river systems in 22 European countries. The analyses show a significant increase in biodiversity over this period, but the stagnant trend since 2010 suggests that significant investment is still needed.

According to the authors, targeted efforts are needed to prevent wastewater treatment plants from overflowing during heavy rainfall and to more effectively remove pollutants from freshwater systems. In addition, the research team advocates restoring freshwater ecosystems by drastically reducing fertiliser and pesticide inputs from agricultural land, better connecting rivers and floodplains, and adapting our river systems to future climate conditions.

"To restore the positive trend of biodiversity recovery, an approach at the scale of whole landscapes is promising," says Verdonschot. "Measures to reduce pollution or improve water retention are only effective if they are applied on a large scale: in the river basin. This means that we need to rethink land use and water management in these areas.