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21 October, 2025

A build-up of ultra-fine sediment in riverbeds is creating oxygen-starved conditions that threaten fish spawning grounds and freshwater invertebrates, new research on the River Taw in Devon has revealed.

A team of Rothamsted scientists found that the smallest sediment particles trapped in riverbed gravel can exert a disproportionally high “oxygen demand” as organic material within them decomposes, depriving eggs and larvae of vital oxygen. The problem appeared to be most acutely related to the very smallest particles in the <25μm fraction of sediment — fine material that settles into riverbed pores and is difficult to flush out.

The study, which analysed sediment oxygen demand (SOD) across nine sites on the Taw, from upland grassland to lowland farmland receiving wastewater discharges, showed consistently higher oxygen demand in riverbed sediments than in their upstream sources. Researchers concluded that trapped algal material within these ultra-fine particles was the likely culprit.

A combined approach — reducing nutrient inputs to rivers and reducing fine sediment losses from land, will be needed to restore healthier habitats.

These fine deposits both block natural pore spaces in the gravel and fuel oxygen depletion, creating a double threat to aquatic life.

One key finding of the study was that oxygen demand of the sediment did not vary when moving downstream to more intensively farmed land, so increased oxygen demand in channel bed sediments couldn’t be clearly linked to the sediment sources present within the catchment. Catchment management therefore has to reduce the nutrient concentrations within the streams to limit algal growth or limit the amount of coarser sediment on the bed which forms a matrix in which the ultra-fine fraction can be trapped.

The findings suggest that current approaches to catchment management may not deliver the best results unless they target both sediment and nutrient sources on land, and in-channel processes that allow riverbeds to “breathe.”

“Reducing in-stream productivity is likely to be very difficult,” said Dr Simon Pulley who led the study, “so a combined approach — reducing nutrient inputs to rivers and limiting the ability of ultra-fine material to be retained on the channel beds by reducing fine sediment losses from land and channel banks will be needed to restore healthier habitats.”

The work highlights the complex links between land use, sewage inputs, and river ecology at a time when freshwater quality in England is under intense public and political scrutiny.

Main image: Shutterstock

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Contacts

Dr Simon Pulley

Scientific Technician - Hydrology

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Rothamsted Research is the longest-running agricultural research institute in the world. We work from gene to field with a proud history of ground-breaking discoveries in areas as diverse as crop management, statistical interpretation and soil health. Our founders, in 1843, were the pioneers of modern agriculture, and we are known for our imaginative science and our collaborative approach to developing innovative farm practice.
Through independent research, we make significant contributions to improving agri-food systems in the UK and internationally, with economic impact estimated to exceed £3 bn in annual contribution to the UK economy. Our strength lies in our systems approach, which combines strategic research, interdisciplinary teams and multiple partnerships.
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We are strategically funded by the Biotechnology and Biological Sciences Research Council (BBSRC), with additional support from other national and international funding streams, and from industry. We are also supported by the Lawes Agricultural Trust (LAT).

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The Biotechnology and Biological Sciences Research Council is part of UK Research and Innovation, a non-departmental public body funded by a grant-in-aid from the UK government.
BBSRC invests to push back the frontiers of biology and deliver a healthy, prosperous and sustainable future. Through our investments, we build and support a vibrant, dynamic and inclusive community which delivers ground-breaking discoveries and develops bio-based solutions that contribute to tackling global challenges, such as sustainable food production, climate change, and healthy ageing.
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ABOUT LAT

The Lawes Agricultural Trust, established in 1889 by Sir John Bennet Lawes, supports Rothamsted Research’s national and international agricultural science through the provision of land, facilities and funding. LAT, a charitable trust, owns the estates at Harpenden and Broom's Barn, including many of the buildings used by Rothamsted Research. LAT provides an annual research grant to the Director, accommodation for nearly 200 people, and support for fellowships for young scientists from developing countries. LAT also makes capital grants to help modernise facilities at Rothamsted, or invests in new buildings.