A 42-year-long study carried out in 14 streams in the Cambrian Mountains has revealed the catastrophic effect of warming water temperatures on Wales’ river life.
The Llyn Brianne Stream Observatory in Powys was launched in 1981. One of the longest running such projects in the world, it has become a key source of evidence and policy advice on river ecosystems.
Led by Professor Steve Ormerod, of Cardiff University’s School of Biosciences, it has observed the impact of rising temperatures on invertebrate life, and the consequent negative impact on larger species such as freshwater fish and river birds like the dipper.
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The observatory has also looked at the impact of drought on water life – prescient given this year's significant lack of rainfall – and the impact of pollution such as agricultural run-off and pharmaceuticals in rivers such as the Wye and the Taff.
The observatory has identified a troubling decline in wildlife in the waterways studied over more than four decades. The streams, which feed into Llyn Brianne reservoir, were selected to represent a variety of habitats.
Some flow through hills grazed by sheep, others through conifer forests - where water acidity levels tend to be high - and others flow through broadleaved woodland. The study area is representative of the rest of upland Wales.
Although water acidity has a negative impact on biodiversity, even the streams relatively unaffected by acidity have shown a shocking decline in invertebrate species, which form a crucial part of the food chain.
Prof Ormerod's studies pinpoint warming water temperatures, as a consequence of climate change, as the pivotal issue. The study has also shown how Atlantic pressure systems are affecting the upland areas of Wales.
These weather systems produce warmer, damper winters - rather than the colder drier winters that were more commonplace even in the relatively recent past – and are having a profoundly negative impact on biodiversity.
Prof Ormerod said: "Many people are unaware of this ongoing tragedy hidden beneath the water surface. Our results show that the build-up to species extinction can start in a subtle way, for example, where climate change causes numbers to decline before sudden disappearance.
"Warm, wet winters, caused by Atlantic pressure systems, synchronise with reductions of abundance and richness in our streams."
Streams in the Llyn Brianne Stream Observatory project are now three degrees centigrade warmer than at the start of the study, with invertebrates declining by one fifth with every degree increase in temperature.
Prof Ormerod says the observatory's findings demonstrate the urgent need for action to mitigate the impact of global warming on Welsh waterways and their wildlife. One solution is to plant broadleaved trees such as alder and willow on riverbanks.
This is critical to the recovery of waterways, providing shade to help prevent further warming in streams and additional nutrients through leaf-fall into the water.
Such planting is also essential for sequestering carbon – broadleaved trees sequester more carbon than conifers. Planting will also protect river banks from erosion by livestock and humans, and will help prevent sedimentation increasing in waterways.
Prof Ormerod added: "Protecting, planting and restoring native plants and their habitats alongside rivers provides food for aquatic organisms and helps prevent high temperatures in the fight against global warming. This step is vital for recovery to begin."
Ruth Jenkins, head of natural resource management at Natural Resources Wales, said: “We know that Wales is being affected by the global climate and nature crises, and that the impacts of these are felt in our rivers.
“The State of Natural Resources Report in 2020 noted that freshwater ecosystems in Wales are facing multiple pressures including climate change, rural and urban pollution, physical modifications, changes to flow, and invasive non-native species.
“Climate change is an increasing pressure and is a serious threat to rivers though exacerbating flood and drought risk, raising temperatures and intensifying other issues.
“For example, storms increase soil erosion and other pollutants in surface run-off in rural and urban areas, as well as overwhelming sewage treatment facilities and remobilising legacy pollutants especially metals in river systems and from moorland peats.
“Woodland creation along rivers and streams can provide significant mitigation through providing shading to keep waters cooler as well as improving water quality through buffering the potential impacts of other land use, such as nutrients and sediment. It also provides food for invertebrates and protects river banks from erosion.
“We are promoting tree planting by supporting the Welsh Government funded schemes which include tree and woodland planting along rivers where appropriate and beneficial. The recent period of prolonged warm and dry weather is further evidence of how our rivers and fish populations are at increased risk from climate change impacts.
“As well as invertebrates, salmon and sea trout are vulnerable to the implications of climate change. This is in part because a warming climate alongside poor water quality has the potential to damage salmonid freshwater habitats and to interfere with their reproductive biology.
“Modified flows may lead to difficulties in downstream seaward migration of smolts and in the re-entry to rivers of adult fish.”
A Welsh Government spokeswoman said: “This week we published the recommendations of a Biodiversity Deep Dive, compiled by independent experts, who have outlined action we can take to address the climate and nature emergencies so as to mitigate the decline of our ecology in Wales.
“The Minister [Julie James] has said she will take forward all of the recommendations of the Deep Dive to restore habitats so that plants and animals are able to adapt to climate change.
“Restoring our rivers will be a key part of this, as will be the Future Sustainable Farming Scheme to help farmers make the right biodiversity choices.
“What’s more, the Welsh Government’s new Small Grants- Woodland Creation scheme offers land managers funding for planting broadleaf trees near rivers and streams.”
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