Water quality in the River Wye catchment will not improve by focusing solely on managing the level of phosphate in the water, according to a study.

The new report, prepared over two years by researchers at Cardiff University for the Wye and Usk Foundation, shows current phosphate levels are mostly within Special Areas of Conservation targets, lower than historical records and unlikely to be the primary cause of algal blooms in the river.

Instead, the researchers found a highly diverse and variable community of diatoms, green algae and cyano-bacteria – often referred to as blue-green algae. Variations in levels of phosphorus and nitrogen across the catchment were also reported in the study.

Increasing levels of ammonium and nitrate, seasonal changes to the river’s flow and high summer temperatures are all combining to impact on the Wye’s health, the report concludes.

Its authors say a holistic management approach, which addresses the river’s flow rate, water temperature, and reduces all nutrients from all sources is essential to reverse the decline in the river’s health.

One of the report's authors Professor Rupert Perkins, of Cardiff University’s School of Earth and Environmental Sciences, said: “Phosphate is seen as a 'low-hanging fruit', an easy thing to focus on as the cause of poor water quality. But it’s just one piece of the jigsaw puzzle.

“By studying the biology with eDNA, alongside water quality measurements, we get a far better understanding of the range of causes behind the problems in the river.”

Between June and November in 2022 and 2023, the team collected 365 samples from 14 different sites along the 200km river – the UK’s fifth largest – spanning parts of Wales and England, which were then analysed at Cardiff University.

Thom Bellamy, another of report’s authors and a PhD student at Cardiff University’s School of Earth and Environmental Sciences, said: “What has changed is the level of nitrate we detected which, for the two-year sampling period, was higher than previously reported. This demonstrates more than anything else that we must look beyond phosphorous levels and monitor the whole suite of nutrients in the river.”

The report is the latest development in a partnership between Cardiff University and WUF, which began in 2018.