Mayflies, stoneflies, and caddisflies are among the diverse group of invertebrates that spend a large part of their lives in the water as larvae. These and many other invertebrates play crucial roles in freshwater ecosystems. They're vital for organic matter decomposition, water filtration, and nutrient exchange between aquatic and terrestrial realms. “Such invertebrates have long been a cornerstone of water quality monitoring,” explains the study’s first author, Prof. Dr. Peter Haase of the Senckenberg Research Institute and Natural History Museum in Frankfurt. “Such monitoring is immensely important because rivers and lakes are subject to major anthropogenic pressures and are among the ecosystems most threatened by biodiversity loss.”

Stressors hinder recovery

Inland waters are exposed to various anthropogenic pressures from agricultural and urban land use. They accumulate pollutants, organically contaminated runoff, fine sediments, and pesticides. In the 1950 – 1960s, water quality in many rivers was very poor due to high pollution levels, for example caused by the untreated discharge of wastewater into rivers. This led to the disappearance of many freshwater species. Water quality improvements beginning in the ’80, especially the construction of waste water treatment facilities, led to considerable water quality improvements.

Establishment of the Water Framework Directive in 2000 led to the implementation of restoration measures aiming at restoring rivers to a good ecological status which aided recovery. “But the recovery was less in urban and agricultural areas and when dams were present,” says Ralf Verdonschot, author from Wageningen University & Research. “The biodiversity increase ceased around 2010. Ongoing stressors, such as still too high nutrient levels and water abstraction, and emerging stressors such as new chemical pollutants and more climatic extremes hinder further recovery.”

Improve wastewater management

The team analyzed a comprehensive dataset of 1,816 time series collected between 1968 and 2020 in river systems in 22 European countries. The analyses show significant increases in biodiversity in this period, but the stagnant trend since 2010 suggests substantial investments are needed.

Targeted efforts are needed to prevent wastewater treatment plants from overflowing during heavy rainfall, and to more effectively remove pollutants from freshwater systems, the authors say. In addition, the research team advocates for freshwater ecosystem recovery through reducing the input of fertilizers and pesticides from agricultural land, connecting the rivers and its floodplains, and adapting our river systems to future climatic and hydrological conditions.

“To reestablish the positive trend of biodiversity recovery, an approach operating on the scale of entire landscapes is promising,” says Verdonschot. “For example, measures decreasing diffuse pollution inputs or improving water retention capacity are only effective when applied on a large scale: the rivers’ catchment. This means that we have to rethink land use and water management in these areas.”