Research and analysis

Exploring phosphorus concentrations and ecological improvements in English rivers: summary

Published 2 March 2026

Applies to England

1. Chief Scientist’s Group report summary

This project investigated changes in phosphorus concentrations and river health in England since 8th January 1980. National river water quality and biological monitoring datasets were analysed to explore the relationship between phosphorus and ecological indicators of river health.

Ecological responses to reduced phosphate concentrations are complex and often confounded by multiple site-specific factors such as river flow and the presence of other pollutants. This project provides evidence to better understand why ecological recovery from phosphorus pollution is not yet evident in some rivers, despite significant reductions in phosphate.

1.1 Background

Phosphorus is a nutrient essential for the growth of aquatic plants, but too much phosphorus is detrimental to the health of freshwater ecosystems. Excess phosphorus in rivers is a significant factor in failure to meet Good Ecological Status under the Water Framework Directive (WFD).

Investments in phosphorus removal technology at sewage treatment works (STWs) have reduced inputs of phosphorus into England’s rivers since the 1980s. However, the extent to which these reductions in phosphorus have resulted in ecological improvements in aquatic ecosystems nationally, is unclear.

1.2 Approach

Associations between observed reductions in phosphorus and metrics of ecological health were examined using Environment Agency monitoring data collected between 8th January 1980 and 27th December 2019. Data beyond 2020 were excluded to avoid changes in data availability due to the impact of the Covid-19 pandemic on sampling and analysis. Aquatic plants (macrophytes), algae (diatoms), and phytoplankton biomass (chlorophyll) were used as biological indicators of river health due to their sensitivity to nutrients. Phosphate concentrations were used to analyse trends in phosphorus, due to phosphate being the most biologically available form of phosphorus. The analysis used multiple datasets, each with a different number of co-located phosphate and biological data river monitoring sites, up to a maximum of 1034 locations.

1.3 Results

Across England, reductions in phosphate concentrations over time were more evident than improvements in ecological health. Compared to the other organisms examined, the clearest response to reducing phosphate concentrations was observed in aquatic plants. An increase in the number and type of species in aquatic plant communities was commonly associated with a significant reduction in phosphate. Reductions in chlorophyll-a concentration were also commonly associated with significant phosphate reductions, suggesting that phytoplankton biomass had become limited by nutrient availability at these sites.

A delay in the biological response to reducing phosphate concentrations was observed in all groups of organisms studied but the magnitude of delay varied by group. The number of aquatic plant species observed in a community was likely to increase before the type of species that make up the community changed. The likelihood of ecological improvement was found to increase with the magnitude of phosphate reduction.

The analyses did not directly assess the effect of phosphorus reduction interventions at STWs on phosphate concentrations and biological metrics. However, the results suggest that improvements in both phosphate and the River Macrophyte Nutrient Index (RMNI) are associated with interventions at STWs. These improvements largely occurred in rivers designated as Eutrophic Sensitive Areas. In these areas, greater phosphorus reduction at STWs is legally required to meet more stringent effluent water quality standards compared to elsewhere. Improvements in the RMNI are indicative of ecological recovery through a shift towards the presence of nutrient sensitive plants and away from the dominance of nutrient tolerant plants.

1.4 Conclusions

Ecological responses to reduced phosphate concentrations are complex and often confounded by multiple site-specific factors such as river flow and the presence of other pollutants. The results show that:

• The magnitude of change in phosphate concentration influences the response of aquatic organisms
• Biological responses to reducing phosphate concentrations are delayed
• Aquatic organisms and communities respond differently to reducing phosphate concentrations.

Ecological improvements in aquatic plant communities have been observed in response to reductions in phosphorus pollution. These findings also help the Environment Agency to understand why ecological recovery is not yet evident in some rivers, despite significant reductions in phosphate. Continued investment in phosphorus management is needed to drive progress towards achieving good ecological status in rivers under the WFD.

1.5 Publication details

This summary relates to information from project SC240019, reported in detail in the following output:

• Report: SC240019/R1
• Title: Exploring phosphorus concentrations and ecological improvements in English rivers
• Project manager: John Robotham, Chief Scientist’s Group
• Research contractor: University of Stirling

This project was commissioned by the Environment Agency’s Chief Scientist’s Group, which provides scientific knowledge, tools and techniques to enable us to protect and manage the environment as effectively as possible.

Enquiries: research@environment-agency.gov.uk

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