Water quality in the River Wye catchment will not improve by focusing solely on managing the level of phosphate in the water, a new study shows.
The chemical, which finds its way into the river from a range of sources, has been linked to a perceived increase in frequency and severity of algal blooms, which are harmful to the river's ecology, wildlife, and those using the river for fishing and swimming.
But the new report, prepared over two years by researchers at Cardiff University for the Wye and Usk Foundation (WUF), shows current phosphate levels are mostly within Special Areas of Conservation (SAC) 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.
Professor Rupert Perkins of Cardiff University's School of Earth and Environmental Sciences and one of the report's authors, 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.
The samples were analysed in labs at Cardiff University to produce a biological fingerprint known as environmental DNA (eDNA) of the cyanobacteria present in the water.
Thom Bellamy, another of report's authors and a PhD student at Cardiff University's School of Earth and Environmental Sciences, added: "This process of extracting the eDNA is important because it gives us an understanding of how the different species are working together in the river.
"Our analysis showed no single species dominance but instead a diverse composition comprising diatoms, green algae and a low level of cyanobacteria. Our eDNA data also corresponds with historical algal surveys conducted on the Wye, suggesting on this measure at least there has been no significant change."
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.
"We're not saying we can forget about phosphorous altogether," Professor Perkins warns.
"We must look at all nutrients as well as flow rates, temperature and the biology of the Wye, working in the catchment on precautionary and preventative approaches."
Generating an action plan that deals holistically with the range of causes that affect water quality will help organisations like Natural Resources Wales and the Environment Agency in river management on the Wye and on a much wider scale too. It's a beautiful river with lots of great wildlife but we need to look at the bigger picture and not silver-bullet quick fixes.
The report is the latest development in a longstanding partnership between Cardiff University and WUF, which began in 2018 as part of an award-winning citizen science project.
Simon Evans, Chief Executive of the Wye and Usk Foundation, added: "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 in the catchment. This didn't make sense. And so, to better understand what was going on we instigated and funded this study."
It shows the current focus on phosphate is misplaced, and we need to manage flow, reduce water temperature, shade our rivers and set science-based thresholds for all nutrients.
