New study published in Nature shows restoration of European rivers might have lost momentum

An article in the scientific journal Nature, entitled “The recovery of European freshwater biodiversity has come to a halt”, highlights a collaborative effort by an international team, which includes two researchers – Alain Dohet and Lionel L'Hoste – of the Environmental Research and Innovation department at the Luxembourg Institute of Science and Technology (LIST), to investigate the status and progression of biodiversity in European freshwater bodies, focusing on invertebrates as indicators. Published this week, their research reveals a notable upswing in biodiversity across river systems spanning 22 nations since 1968. However, the team of scientists warns that this positive trend has stagnated since 2010 and that many river systems have not been able to fully regenerate. Therefore, they call for additional measures to revive the recovery of biodiversity in inland waters – freshwater ecosystems that are and continue to be exposed to serious pressures such as pollution, invasive species, and climate change.

First recovery, then stagnation

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 contribute to important ecosystem processes in freshwater bodies. They decompose organic matter, filter water and transport nutrients between aquatic and terrestrial environments. In addition, such invertebrates have long been a cornerstone for monitoring water quality", explains the first author of the study, Prof. Peter Haase of the Senckenberg Research Institute and Natural History Museum in Frankfurt.

Inland waters are exposed to various anthropogenic pressures due to agricultural and urban land use. They accumulate pollutants, organically contaminated runoff, fine sediments and pesticides, and are also threatened by changes such as the construction of dams, water withdrawal, invasive species and climate change. In response to the poor condition of bodies of water in the 1950s and 1960s, countermeasures were implemented to restore freshwater habitats.

Over the past 50 years, these steps have contributed to the containment of wastewater pollution and thus to improvements in freshwater biodiversity. Nevertheless, the number and impact of stressors threatening these ecosystems continue to increase worldwide and the biological quality of rivers remains inadequate in many places.

"Luxembourg in no exception. Albeit considerable efforts to reduce organic pollution, driven mainly by improving wastewater treatment, on average the required ‘good’ ecological status has still not yet been achieved," explains Alain Dohet, conservation biologist and lead researcher at LIST.

Together with a large international team, a comprehensive dataset was investigated consisting of 1,816 time series collected between 1968 and 2020 in river systems in 22 European countries, comprising 714,698 individuals of 2,648 taxa from 26,668 samples. The analyses show significant increases in biodiversity over the 53- year period. The number of unique taxa increased 0.73 percent per year, the number of groups of taxa performing different ecological roles increased 2.4 percent per year, and total abundance of invertebrates increased 1.17 percent per year.

However, most of these increases happened before 2010, and unfortunately, biodiversity has remained relatively stable since then, explains Dohet. The improvements in biodiversity during the 1990s and 2000s were due to better water quality and ecological restoration efforts. But the fact that progress has slowed afterward suggests that previous actions have reached their limit.

According to the study results, freshwater communities downstream of dams, urban areas and farmland recovered less rapidly. Fauna at sites with faster warming also recorded lower increases in species diversity, abundance of individuals and functional diversity.

Possible actions going ahead

Substantial investments are needed to expand wastewater networks and improve wastewater treatment plants. Targeted efforts are needed to prevent wastewater treatment plants from overflowing during heavy rainfall, and to more effectively remove micropollutants, nutrients, salts, and other 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 floodplains to reduce destructive flooding, and adapting our river systems to future climatic and hydrological conditions.

“These management actions should target sites at greater risk of biodiversity decline, such as those downstream of urban areas, cropland and dams, while maintaining and strengthening protection of the least impacted systems that are refuges of biodiversity (e.g., headwater streams). Further work to understand the causes of the stalling of biological recovery in Europe’s rivers, would help to guide management actions maximizing the protection of biodiversity. In the coming months, our team will mainly focus on the modifications of community composition in relation to climate warming (e.g., replacement of cold-adapted species by thermophilic and generalist species) and the influence of the upstream–downstream gradient in structuring these communities”, concludes Alain Dohet.