Efforts to fix the declining health of the river Wye have focused too much on one particular problem and have overlooked others, according to researchers.

There has been a reported increase in the frequency and severity of “algal blooms” in the river, which flows from the Welsh mountains through Herefordshire to the Severn estuary.

These are harmful to the river’s wildlife and to people using the river for fishing and swimming. It was thought they were mainly caused by phosphates, partly from fertiliser which had washed off fields in the river catchment.

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But now a new report by researchers at Cardiff University, funded by local conservation group the Wye and Usk Foundation (WUF), shows phosphate levels in the river are actually lower than shown in historical records, and are unlikely to be the main cause of the blooms.

Instead, they concluded that increasing levels of ammonium and nitrate, along with seasonal changes to the river’s flow and high summer temperatures, are all combining to impact on the Wye’s health – and that a more holistic approach is now needed to reverse its decline.

One of the report’s authors, Professor Rupert Perkins, said: “Phosphate is an easy thing to focus on as the cause of poor water quality. But it’s just one piece of the jigsaw puzzle.”

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Fellow author Thom Bellamy added: “What has changed is the level of nitrate we detected which, for the two-year sampling period, was higher than previously reported.

“We must look beyond phosphorous levels and monitor the whole suite of nutrients in the river.”

Explaining what prompted the study, WUF chief executive Simon Evans said: “While phosphate levels in the Wye have been declining, in large part due to investment by Dŵr Cymru Welsh Water, we were still receiving reports from members of the public that algal blooms had been spotted and were getting worse.

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“This didn’t make sense.”

He said the report now shows the current focus on phosphate in the river is “misplaced”.

“We need to manage flow, reduce water temperature, shade our rivers and set science-based thresholds for all nutrients,” he said.