A curious finding: why are phosphate levels so low in our rivers?

New findings from Earthwatch’s Great UK WaterBlitz

raise some interesting questions about phosphate pollution, wastewater treatment and the health of our freshwater environments.

An intriguing message arrived from Earthwatch this morning, following its Autumn Great UK WaterBlitz. The results, it seems, have thrown up something rather unexpected: Phosphate concentrations were abnormally low compared with previous autumn WaterBlitz events.

Earthwatch was unable to establish the cause and has therefore decided to exclude phosphate from its analysis, concentrating instead on nitrate concentrations.

This caught our attention at New Forest Water Watch (NFWW), because consistently low phosphate readings are something we have been observing in our own monitoring across the New Forest.

Could our local findings be part of a much wider national picture? And, if so, what might be happening?

A long-term improvement in river phosphate levels. There is some encouraging background to this story.

Phosphorus is an essential nutrient for aquatic life, but excessive amounts entering rivers and streams can cause serious problems. High phosphate concentrations can encourage excessive growth of algae and aquatic plants. When this growth dies back and decomposes, oxygen is consumed, potentially harming fish and other aquatic organisms. This process is known as eutrophication.

For many years, sewage discharges and agricultural run-off have been major sources of phosphorus pollution in our rivers. However, there has been a significant change over recent decades.

In March 2026, the Environment Agency published a research report, Exploring phosphorus concentrations and ecological improvements in English rivers. Its analysis of long-term monitoring data found substantial reductions in phosphate concentrations in English rivers since the 1980s.

One important contributor has been investment in improved phosphorus-removal technology at sewage treatment works.

This is a significant development. It suggests that, in many places, wastewater treatment is now much more effective at removing phosphorus before treated water is discharged into rivers.

What are sewage treatment works doing differently?

There are two main approaches to removing phosphorus during wastewater treatment.

Chemical removal

Chemicals containing iron or aluminium are added to wastewater. These react with dissolved phosphate to form compounds that can be separated from the water and removed with the sewage sludge.

Biological removal

Certain bacteria can be encouraged to absorb and store phosphorus within their cells. These bacteria are then removed as part of the treatment process. Both methods can substantially reduce the amount of phosphorus discharged into receiving rivers.

This is an important distinction when considering the quality of treated wastewater. A treatment works may now remove phosphorus very effectively, but that does not necessarily mean it removes every other pollutant to the same degree.

In other words, a discharge can have very low phosphate concentrations and still contain other contaminants.

Why were this autumn’s results so unusual?

The long-term reduction in phosphate concentrations is well documented. But it doesn’t necessarily explain why Earthwatch found unusually low readings in its latest autumn survey.

We can think of several possible explanations.

1. Continuing improvements in wastewater treatment

Further investment in phosphorus removal, together with changes in agricultural practices, may be contributing to declining nutrient inputs. This may be a particular issue outside the New Forest.

2. Rainfall and river conditions

River flow, rainfall and dilution can all influence measured concentrations. The timing of sampling in relation to rainfall and pollution inputs can make a considerable difference to the results.

3. Biological uptake

Aquatic plants and algae take up dissolved phosphate as they grow. This can temporarily reduce the amount remaining in the water, particularly during periods of active growth.

4. An analytical issue

This is perhaps the most important possibility to investigate. If phosphate readings were unexpectedly low across a very wide geographical area, it would be sensible to check whether there had been any changes in testing kits, reagents, calibration or analytical procedures.

Earthwatch has been appropriately cautious in excluding phosphate from its analysis rather than drawing conclusions from results it cannot yet explain.

It will be interesting to see whether further investigation identifies a genuine change in environmental conditions or a problem with the measurements. Low phosphate does not necessarily mean clean water any more than clear water means clean water

There is another important consideration, particularly for those of us monitoring water quality in the New Forest.

Phosphate and E. coli tell us very different things about the water. Like Nitrate, Phosphate is a nutrient. Its concentration reflects inputs from the surrounding catchment, treatment processes, dilution and biological uptake. E. coli, on the other hand, is an indicator of faecal contamination.

Ammonia can indicate sewage or agricultural pollution and, at sufficient concentrations, can be toxic to aquatic life. Dissolved oxygen provides further information about the conditions experienced by aquatic organisms. These parameters need to be considered together.

Our NFWW monitoring has demonstrated that very high E. coli concentrations can occur alongside consistently low phosphate readings. There is no contradiction here. A stream can have little measurable phosphate and still be receiving faecal pollution.

This is one of the reasons why monitoring several different water quality parameters is so valuable. No single test can tell us everything about the health of a river.

What does this mean for the New Forest?

Many New Forest streams have relatively low nutrient concentrations, reflecting their natural catchment characteristics and, in some cases, the nutrient-poor environments through which they flow.

Our consistently low phosphate readings may therefore reflect a combination of naturally low background concentrations and wider improvements in phosphorus removal. It is too early to say how much each factor contributes.

What would be particularly interesting is to compare our New Forest results with those from other parts of the country, and to examine whether the latest Earthwatch findings represent a continuation of the long-term national decline or something quite different.

Earthwatch’s findings raise some interesting scientific questions, e.g. …

Were phosphate concentrations unusually low across all four UK nations, or were there regional differences?

How many readings were close to, or below, the detection limits of the testing kits? This is something we think about.

Were the same testing methods, reagents and procedures used as in previous autumn surveys?

Could changes in wastewater treatment and agricultural practices explain some of the differences?

These are important questions if we are to understand what the results really mean. An encouraging development, but a reminder to keep looking!

The long-term reduction in phosphate concentrations in English rivers is encouraging. Improved phosphorus removal at wastewater treatment works has clearly contributed to this progress.  But we should not confuse progress in one area of water quality with an overall improvement in river health. Sewage pollution, faecal contamination, ammonia, chemicals and other pollutants remain important concerns. And even where phosphate concentrations have fallen, ecological recovery can take time. The Environment Agency’s research highlights that improvements in water chemistry do not always translate immediately into improvements in aquatic plant and animal communities.

For New Forest Water Watch, the message is clear: keep monitoring, keep measuring and keep asking questions.

Sometimes, as this latest Earthwatch announcement demonstrates, the most interesting findings are the ones that leave us with more questions than answers.