New research shows that rising water temperatures and increased nutrient levels pose serious threats to freshwater ecosystems.

Scientists previously assumed that rising temperatures caused by climate change and increases in nutrient levels caused by pollution could offset each other. But a new study has shown that warming waters and nutrient overload are a dangerous combination that threatens our rivers and lakes. The work was published in the journal Ecology Letters.

The study, conducted by an international team of scientists from the universities of Sheffield, Oxford, Savoie Mont Blanc and the French National Research Institute for Agriculture, Food and the Environment, examined hundreds of lakes and rivers and analyzed the complex relationships between fish species.

Researchers have found that food webs, the complex networks that determine who eats whom, become less complex in warmer, more nutrient-rich waters. This simplification leads to shorter food chains and a poorer functioning ecosystem. Top predators are particularly vulnerable to these changes, but are essential to ecosystem functioning and stability.

Less than 3% of the water on our planet is freshwater, yet freshwater habitats are home to nearly 10% of all known animals and nearly half of all known fish species. In addition to helping maintain water quality on land and in the sea, freshwater ecosystems enable wildlife such as eels and salmon to travel long distances to complete their life cycles and act as conveyor belts for transporting nutrients that make soil palatable for growing food.

The far-reaching impact of the need to reduce pollution levels in our rivers was felt worldwide when the Olympic open water swimming events such as the triathlon, which were due to take place in the Seine in Paris, had to be postponed due to harmful levels of bacteria. The study highlights that this need is even more urgent given the increasing effects of warming caused by climate change, but also suggests that reducing pollution levels is a promising way to mitigate the effects of climate change.

Alain Danet, a postdoctoral researcher at the University of Sheffield and co-lead author of the study, said: “The eyes of the world were on the Seine this summer because of the Olympic Games. The challenges in Paris show how important it is to reduce pollution in our rivers and lakes – not only for our own health, but also for the health of our ecosystems.”

“Our rivers, lakes, wetlands and underground water supplies store and purify the water that is vital to humans and animals. From providing drinking water to supplying terrestrial and marine ecosystems with vital resources, healthy freshwater systems are critical. These vital ecosystems also protect us from flooding, filter pollution and support countless plants and animals.”

Willem Bonnaffé, a postdoctoral fellow at the University of Oxford and co-lead author of the study, said: “Lake and river life are like the kidneys of our planet. They purify the water and prevent the blooms of toxic algae and bacteria. If we protect them, we protect ourselves.”

“Climate change and pollution are not isolated problems for our rivers and lakes, as scientists previously believed. Our study found that they interact in complex ways and threaten the balance of life in our waters. By reducing greenhouse gas emissions and improving water quality, we can help preserve the health of our rivers and lakes for future generations.”

Camille Leclerc, a postdoctoral researcher at the French National Research Institute for Agriculture, Food and the Environment and co-lead author of the study, said: “This important research has shown what a dangerous combination warming waters caused by climate change and increases in nutrient levels caused by pollution are.”

“More positively, however, this study shows that if we can reduce their pollution, our lakes and rivers will be better protected from the effects of climate change and warming waters.”