Mussels are disappearing from the Thames and getting smaller – partly because the river is cleaner

Freshwater ecosystems, including rivers, are home to 10% of all known animal species. But at the same time, they are losing their species diversity faster than any other ecosystem type worldwide. Because animal species respond in different ways to different threats, it is difficult to assess the health of these river systems.

But the population status of species such as freshwater mussels can reveal broader trends in the ecosystem. Freshwater mussels live in riverbeds and feed by filtering algae and other organic particles from the water. As they settle into the river bed and remain mostly stationary, they are exposed to many of the stressors that threaten rivers and are therefore a useful indicator of the health of the river in which they live.

Mussels also serve as ecosystem engineers. They keep the water clear and prevent the development of harmful algal blooms. They also promote freshwater biodiversity by providing habitat and nutrients to riverbed invertebrates. Freshwater mussels are among the most endangered animal groups in the world. Yet in Britain we have little information on the health of these species.

I participated in a recent study evaluating the population status of freshwater mussels in the River Thames. We observed an alarming decline in the number and size of mussels, which could harm the health of the river ecosystem. But some of the changes we observed may be the result of efforts to return the River Thames to a more “natural state”.

The mussels of the river are endangered

An influential study conducted in 1964 underpins much of our understanding of freshwater mussels in the River Thames. The survey was one of the first to quantitatively evaluate freshwater mussel populations. It was performed at a site near Reading by Christina Negus, then a University of Reading postdoctoral researcher.

Her research showed that freshwater mussels represent 90% by weight of the living organisms in the Thames riverbed. The study’s findings highlighted the role of freshwater mussels as some of the most important species in the river.

Our study reassessed the population of freshwater mussels along the same stretch of river, using methods identical to Negus’s. We found that the freshwater mussels population has declined by almost 95% since 1964. One species, the depressed river mussel, may have completely disappeared from the river.

The results of our study also suggest that River Thames mussels are smaller than they were at the time of the original study. Their overall size and growth rate have decreased by 10%-35% compared to 1964.

We also identified the presence of an invasive mussel species, the zebra mussel. Zebra mussels are found throughout Europe and North America and threaten native mussel species by nesting directly on their shell, competing for food and sometimes preventing a mussel from opening. The presence of the highly invasive zebra mussel, which was not observed in the 1964 study, may have contributed to the decrease in the total number of freshwater mussels recorded by our study.

A closed zebra mussel with an orange shell marked with a zebra-like pattern.
The survey recorded the invasive zebra mussel in the River Thames.
RLS photo/Shutterstock

A cleaner river

Lower levels of nutrients, such as phosphate, in the river may be part of the reason why clams are smaller and grow more slowly than observed in 1964. Nutrients encourage the growth of algae, an important food source for clams. A reduced nutrient content could therefore lead to lower food availability for mussels and, as a result, slower growth.

The Thames was heavily polluted at the time of the original study. We spoke to Negus, who recalled having a sore throat for the entire two years she did her research, a symptom she attributes to the polluted river. This implies that the size and growth rate of freshwater mussels recorded in 1964 may have increased artificially due to nutrient pollution from human sources.

A layer of green algae covering a pond.
Algae are a food source for mussels.
Manishankar Patra/Shutterstock

But since the Thames was declared “biologically dead” in 1957 due to levels of pollution, it has recovered and is now one of the cleanest urban rivers in the world.

Stricter rules on sewage discharge have led to significant reductions in phosphate concentrations in the River Thames since the 1960s. Viewed this way, today’s smaller clams may be an indication of the river’s return to a more “natural” state.

Changing ecosystem

However, the picture is more complicated than this suggests. Invasive species and wider threats to habitat, such as dredging and intensive land use along the riverbank, may also have caused the decline of what were once some of the river’s most abundant animals. So these drops are a warning about the health of the river’s ecosystems.

Among the mussels experiencing the greatest decline are barnacle and painter’s mussel, whose populations have declined by 98.9% and 96.8% respectively. Both species are generally considered common and not considered endangered. As a result, there are currently no monitoring programs or conservation measures for these species.

Several of the mussel species found in the Thames are on grass.
Different types of freshwater mussel found in the Thames: the painter’s mussel, the swollen river mussel and the barnacle.
Author provided

If our findings reflect a wider decline in the status of freshwater mussels in UK rivers, then we could be approaching a critical and unexpected population collapse. Such a collapse is likely to have a negative effect on freshwater ecosystems due to the role of mussels in promoting invertebrate biodiversity.

So while a cleaner river is positive for riverine biodiversity, such severe declines in these once-abundant species suggest that we need to step up our efforts to protect these valuable but fragile ecosystems.

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