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Water pollution makes river biodiversity more vulnerable to climate warming

March 3, 2016
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River Torridge in Devon, a sample site for mayflies in the study. Image: alexwhite | Flickr

Polluted rivers with low oxygen levels are more susceptible to the harmful effects of climate change, according to a new study co-authored by MARS scientist Professor Steve Ormerod.

Researchers from Cardiff University and Radboud University in the Netherlands led by Wilco Verberk used laboratory studies and over 42,000 samples from UK rivers to show that two common mayfly species are less able to tolerate rising water temperatures in polluted rivers with low oxygen levels. The breakdown of organic pollutants such as sewage and farm run-off uses oxygen, meaning that polluted waterways often suffer severe drops in dissolved oxygen levels.

The study, published in Global Change Biology (open access), adds to the growing evidence on the influence of multiple stressors in shaping how freshwater ecosystems are likely to respond to climate change. Specifically, it suggests that reductions in water pollution may help increase the resilience of freshwater biodiversity to the effects of future climate change.

“Cool water insects like many mayflies are in triple jeopardy in warmer, polluted waters,” explains Professor Ormerod. “First, at higher temperatures, water can hold less oxygen. Secondly, insects need more oxygen to keep pace with their needs as temperatures increase. Thirdly, oxygen is used in the breakdown of organic pollution, with this effect occurring most rapidly in warmer waters. These three effects mean that warm polluted waters are the worst combination.”

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Blue-winged olive (Serratella ignita) Image: Wikipedia

In laboratory studies, the team found that mayflies such as the green drake (Ephemera danica) and the blue-winged olive (Serratella ignita) were able to survive temperatures 3-5 °C higher where oxygen levels were high, compared to where they were depleted. Hypoxia – or severe oxygen depletion – lowered lethal water temperature limits by roughly 5.5 °C and 8.2 °C for the green drake and blue-winged olive respectively. Mayflies were used as bioindicators in the study as they are common and ecologically important features of many river and stream ecosystems.

Analysis of long-term field study data provided support for these laboratory findings, showing that mayfly populations were significantly reduced in poorly oxygenated stream sites, and that these reductions were particularly pronounced under warm water conditions. In short, low oxygen levels reduced the optimum stream temperature for mayfly populations, and threatened their abundance. The team used data provided by the Environment Agency and Natural Resources Wales collected using kick samples from 2632 sites across England and Wales between 1989 and 2008.

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Green drake (Ephemera danica). Image: Wikipedia

In bringing together field and laboratory studies as a means of assessing how reductions in pollution may help to adapt river ecosystems to climate change, the study is highly innovative. The broad similarities in findings between the field and laboratory studies suggests that low oxygen levels not only impair mayfly survival at extremes of temperature, but can also restrict their abundance at temperatures well below lethal limits.

Improving water oxygenation through management of pollution could thus provide a key element in strategies to adapt Britain’s rivers and streams to climate change, potentially increasing ecosystem resilience to rising water temperatures in the future.

Co-author Dr Isabelle Durance, Director of Cardiff University’s new Water Research Institute states, “Our work presents real hope in the fight against climate change. We need to find ways to reduce the future effects of warming, and our data show how regulating and reducing pollution offers real benefit.”

Wilco Verberk et al (2016) Field and laboratory studies reveal interacting effects of stream oxygenation and warming on aquatic ectotherms. Global Change Biology, 10.1111/gcb.13240

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