Thames mussel studies 60 years apart show nature decline

An 80-year-old Reading zoologist’s research from almost six decades ago has played a significant role in a ‘very worrying’ discovery about declining biodiversity in the River Thames. 

Tina Negus, from Lincolnshire, surveyed the Thames as a University of Reading zoology postgraduate student in 1964 to observe freshwater mussel populations in the river. Almost 60 years later, Cambridge PhD zoology student Isobel Ollard returned to the same stretch of water in Reading to replicate Tina’s research.

Strikingly, Isobel found that the mussel population in this area had been almost completely wiped out in the intervening six decades. Mussels are an important indicator of the health of a river ecosystem and such a large decline in biomass is likely to have a knock-on effect for other species, reducing the overall biodiversity of the Thames. 

Tina, who left academia to teach ceramics in Birmingham before retiring more than a decade ago, said: “In the past 60 odd years, there's been quite a difference. It must be fairly unusual to have a project start at the same place on the same topic so many years apart where the original researcher is still alive and kicking.

“Despite the results, it was nice to know that it was being carried out using identical techniques after all this time. Maybe in another 50 years, somebody else will carry out the survey in the same area.”

Depressed mussel decline

Isobel found that mussel numbers have declined by almost 95%, with one species – the depressed river mussel – almost completely gone. The mussels that remained were much smaller for their age, reflecting slower growth. These findings were described as “very worrying” by Isobel, as mussels help to filter water, remove algae, and provide places for other aquatic species to live. 

The scientists think the mussels’ reduced growth rate may reflect the river’s return to a more ‘natural’ state, as levels of nitrate and phosphate in the river water have fallen due to tighter regulation of sewage treatment since the 1960s. A reduction in these nutrients would reduce the growth of algae, limiting the food available to the mussels.

The research was made possible by Tina’s past endeavours. Her study, published in 1966, continues to be cited extensively as evidence of the major contribution mussels make to ecosystem functioning in rivers.

‘Clear warning signal’

Isobel said: “Tina’s important research from almost six decades ago was vital in helping us to understand the health of the Thames today.

“This key piece of information has proved really significant in revealing how ecological conditions have changed at this site near Reading, and it acts as a clear warning signal about threats to the world’s freshwaters.”

Isobel contacted Tina to learn more about how she carried out her research and when they spoke, the octogenarian told Isobel about the high level of pollution in the river at the time of her research. Tina was able to confirm that invasive zebra mussels were not present at the site at the time of her study, which was important in identifying that they had colonised the site at some point in the intervening period.

Tina’s childhood interest in natural history led to her choosing to study zoology, botany and geography as an undergraduate at the University of Reading in the 1950s. She then had the opportunity to do a postgraduate degree, including research on freshwater mussels on the River Thames.  

‘Declines in freshwater mussel density, size and productivity in the River Thames over the past half century' is published in Journal of Animal Ecology. 

Ollard, I., & Aldridge, D. C. (2022). Declines in freshwater mussel density, size and productivity in the River Thames over the past half century. Journal of Animal Ecology, 00, 1– 12. https://doi.org/10.1111/1365-2656.13835