A new report from the Environment Agency and the University of Stirling looks at the links between phosphorus concentrations and ecology in English rivers. It offers evidence that reductions in phosphorus pollution in these rivers are associated with ecological recovery, although improvements remain uneven and can take years to emerge.
The study analysed decades of monitoring data to assess how declining phosphate levels relate to biological indicators such as aquatic plants, algae and phytoplankton.
Phosphorus pollution remains one of the most significant causes of ecological degradation in England’s rivers because excess nutrients drive eutrophication – the overgrowth of algae and aquatic plants that harms ecosystem health and water quality.
The report’s authors say phosphorus concentrations have declined in many rivers over recent decades, partly due to the installation of phosphorus removal technology at sewage treatment works (STWs), alongside changes in fertiliser use and agricultural management practices.
Declining phosphorus linked to ecological change
The research analysed Environment Agency monitoring data collected between the 1980s and 2020, linking water chemistry measurements with biological indicators such as macrophytes (aquatic plants), diatoms (algae), and chlorophyll-a as a proxy for phytoplankton abundance.
Across hundreds of monitoring sites, reductions in phosphate were relatively common. Up to 57% of co-located monitoring sites showed significant declining trends in phosphate concentrations, with average reductions of 0.14–0.36 mg/L depending on dataset and biological indicator. However, “significant reductions in phosphate were not detectable in all rivers”, said the document.
Also, ecological improvements were far less widespread, occurring at between 2% and 20% of sites depending on the metric examined.
Where biological improvements did occur, they were most strongly associated with changes in aquatic plant communities. For example, 18.7% of sites showed improvement in the River Macrophyte Nutrient Index (RMNI) and 14.4% showed increases in the number of macrophyte taxa, suggesting a shift towards more nutrient-sensitive plant species.
Reductions in chlorophyll-a were also linked to declining phosphate concentrations, indicating that lower nutrient levels may be limiting the growth of phytoplankton in some rivers.
However, changes in diatom communities were rare, with more than 92% of sites showing no significant change in the metrics examined.
The report highlights a strong association between ecological improvements and rivers located in designated Eutrophic Sensitive Areas, where substantial investment has been made in phosphorus removal at sewage treatment works.
Of 38 sites in these areas, 30 showed improvements in both phosphate concentrations and the River Macrophyte Nutrient Index, indicating a shift towards plant communities typical of less nutrient-rich waters.
This reflects decades of infrastructure investment. Between 1995 and 2020, the water industry spent around £2.1 billion on phosphorus removal at sewage treatment works, contributing to a 66% reduction in phosphorus loads from these facilities over that period.
Recovery can be slow and uncertain
Despite these improvements, the study emphasises that ecological recovery from nutrient pollution is complex and can take time. Biological responses may occur only after phosphorus concentrations fall below certain thresholds, and improvements may be influenced by other environmental pressures such as habitat modification, sediment pollution, nitrate levels or river flow changes.
Monitoring data show that while many rivers have experienced chemical improvements, more than 79% of monitored sites showed no significant change in macrophyte indicators, illustrating the slow pace of ecological recovery.
The findings support the government’s wider policy push to reduce nutrient pollution through wastewater treatment improvements, agricultural regulation and catchment-based management.
Environment Agency Chief Scientist Rob Bradburne said:
“Our scientific evidence and data are crucial to understanding and tracking progress, enabling us to assess the impacts of decades of investment and regulation designed to reduce phosphorus pollution.
“This research confirms that phosphorus reduction in rivers is leading to ecological recovery with aquatic plant life beginning to shift towards a more diverse range of species.
“We need to maintain momentum and continue to bring nutrient levels down through wastewater management, river basin management planning and utilising nature-based solutions.”
Professor Nigel Willby of the University of Stirling added:
“Phosphorus is a significant pressure on freshwaters and sewage treatment is a major source of this phosphorus. Managing nutrient inputs should therefore benefit the ecological health of rivers.
“In this study, using data from hundreds of sites on English rivers with long runs of overlapping data on nutrients and biology, we found encouragingly that phosphorus levels have widely improved on a decadal scale.
“Biological improvements were commoner where phosphorus had also been reduced, often coinciding with the largest reductions in phosphorus, but were patchy and often took years to emerge. Nutrient loads to rivers evidently can be managed to give positive ecological outcomes but these are not quick or guaranteed wins.”
