Long-term or multi-decadal climate change has led to increasing water temperatures and algae levels
Representative photo: iStock Extreme weather events such as droughts and heatwaves led to increased concentrations of pollutants and salinity in rivers across the world, according to a new study.
An international team of researchers reviewed 965 studies on the impact of drought, heatwaves, rainstorms and flooding on river water quality.
They observed water quality deterioration under droughts, heatwaves and compound drought–heatwave events in 68 per cent of the case studies.
The researchers, led by Utrecht University, Netherlands, also analysed long-term (multi-decadal) changes in climate between 2000 and 2022. Their findings were published in the journal Nature Reviews Earth & Environment on September 12, 2023.
Long-term or multi-decadal climate change has led to increasing water temperatures and algae levels in 56 per cent of case studies, the researchers noted.
The scientists looked at multiple markers of water quality, such as temperature, salinity, dissolved oxygen levels, algal blooms and concentrations of nutrients, metals, microorganisms, pharmaceuticals and plastics in river water.
Droughts and heatwaves led to increased salinity and higher concentrations of pollutants, such as pharmaceuticals. And the surge in algae levels has contributed to a decline in dissolved oxygen concentrations in river water, they found.
An estimated five million fish died in the Darling River, Australia, between 2018 and 2019, mainly owing to low dissolved oxygen during a hydrological drought.
Declining river flow during summer can contribute to stronger temperature rises in waterbodies, the study found. The largest water temperature surges are projected for the south-eastern USA, southern Europe, eastern Asia and southern parts of Africa and Australia.
The document also highlighted an overall deterioration in river water quality under rainstorms and floods in 51 per cent of the cases.
Rainstorms and floods generally result in the accumulation of more contaminants in rivers and streams. Increased concentrations of suspended sediments, plastics, nutrients (mainly in particulate forms), some metals, and microorganisms are found after rainstorms and floods.
Furthermore, the sequence of different extreme events (such as droughts followed by floods) also impacts river water quality responses and their driving mechanisms.
Most rivers and streams analysed in this study were from North America and Europe. Existing water quality monitoring data are highly fragmented in several regions, such as Africa and parts of Asia.
There is a need to compile local or regional water quality monitoring data to large data sets and use (large-scale) water quality models, tools and techniques, the study suggested. This will help to strategically select streams and rivers to be prioritised in expanding in situ water quality monitoring campaigns.
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