The composition of diatom communities, used as indicators of stream and river water quality, varies throughout the seasons, a new study by Lancaster University researchers reveals. This means that we may be underestimating the impacts of pollution in our watercourses, because these organisms are generally not monitored in winter when pollution can be at its highest.

Diatoms are single-celled algae that form one part of the stream-bed community. Environmental regulators monitor diatoms in watercourses to provide an indicator for the level of nutrients, especially phosphorus, that may be polluting streams and rivers. These nutrients, which come from many sources including fertilisers used in agriculture and from wastewater, may cause harm to wildlife in rivers and the oceans.

Diatom monitoring is typically undertaken twice a year, in spring and in autumn. But the new study, published in Scientific Reports, shows that the diatom community varies systematically throughout the year and suggests that stream pollution can be particularly bad in winter.

Dr Maria Snell, a former researcher at the Lancaster Environment Centre and lead author of the paper, took monthly measurements in three tributaries of the River Eden in Cumbria over seven years. This work showed a regular, seasonal variation in the makeup of the diatom communities, with these communities indicating significantly more phosphorus in the streams during winter.

“Maria’s work shows that at some sites there was a dramatic variation in the diatom community across a year, a pattern that was repeated through several annual cycles.” said Dr Ben Surridge, a co-author of the study.

“For an individual stream, the diatom community suggested river water quality could go through the full spectrum from high to bad status at different times of year. If a sample was taken at one point in the year you would conclude the stream had a good status, but if you sampled the same stream in winter the status could have dropped substantially and be classified as bad.”

The streams that saw the largest variation in quality through the year drained the most intensively farmed catchments, and the stream with the smallest difference between seasons drained the least intensively farmed catchment.

“Runoff from agricultural land in winter may carry nutrients from soils to streams and rivers, so in this respect the diatom community may be reflecting this process and suggesting greater phosphorus availability in the streams in winter.”

What is particularly worrying, according to the researchers, is that water quality in winter could get worse with climate change.

“There are predictions of wetter winters for the North West of England as climate changes in the future, so potentially the variation in river water quality through the year could become even more pronounced. Monitoring and understanding the condition of streams and rivers in winter is likely to become even more important in the future,” said Ben.

Professor Phil Barker, co-author of the paper and Director of the Lancaster Environment Centre, said: ‘Water quality policies have tended to ignore the crucial winter season and yet we expect UK winters to get wetter in the future. We must work closely with farmers to help reduce pollutant movement from land to water during this critical season, and with governments to ensure climate change is better captured by future legislation.’

The study was part of the River Eden Demonstration Test Catchment (DTC) project, funded by the Department for the Environment, Food and Rural Affairs and led by Lancaster University. The DTC project involved researchers working with farmers and other stakeholders to improve catchment management: a workshop to celebrate and reflect on the achievements of the ten year project was held last week.