A 41-year-long study of streams in the Cambrian Mountains has highlighted “the catastrophic effect” of warming water temperatures on invertebrate life, leading to disappearance and decline, and impacting fish and river birds in the region.

The Llyn Brianne Stream Observatory study, led by professor of ecology at Cardiff University’s School of Biosciences, Steve Ormerod, has been running since 1981 monitoring 14 streams in the Cambrian Mountains, with a shock revelation that in just two decades, the water temperature has risen by three degrees.

The study reveals that the “catastrophic effect” of warming water temperatures on invertebrate life, and the consequent negative impact on larger species such as freshwater fish and river birds like the dipper.

Prof Ormeord said the decline in invertebrates in the region’s streams is a “hidden tragedy.”

The Llyn Brianne Stream Observatory is one of the longest running catchment projects anywhere in the world and has become a key source of evidence and policy advice on river ecosystems – with global significance.

Prof Ormerod, who is also vice-president of the RSPB and deputy chair of Natural Resources Wales, 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 Llyn Brianne Stream Observatory 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 was chosen as 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, speaking to the Cambrian Mountains Society as part of the charity’s campaign to designate this upland region an Area of Outstanding Natural Beauty, 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 said that 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 broad leaved 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 riverbanks 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.”

The Cambrian Mountains Society, formed in 2005, is continuing its campaign to have the Cambrian Mountains region designated as an Area of Outstanding Natural Beauty.

The Cambrian uplands spread across Carmarthenshire, Ceredigion and Powys, and includes exceptional glaciated plateaux, lakes, rivers and valleys and host a wide range of threatened plants and animals – yet are the only area in Wales to have no formal protection whatsoever.

The society said that the designation would “help in the quest to restore this unique upland landscape, including the vital waterways and their biodiversity.”

A proposal in 1972 to designate the Cambrian Mountains as Wales’ first national park lapsed when the order was left unpassed by the Westminster parliament.

AONB designation, unlike a national park, would not involve a separate planning authority but would coordinate management of the region alongside the three county councils.

A petition calling for the region to be protected has attracted more than 3,000 signatures.

Once it has reached 10,000 the issue of designating the area as an AONB will need to be debated in the Senedd.

The petition can be found at petitions.senedd.wales/petitions/­245150