Multiple factors potentially reduced the mussel population in the River Thames by 95%, study says

(CN) — Isobel Ollard, a PhD student in the University of Cambridge's department of zoology, considers a 1964 survey of the mussel population in the River Thames an important source of information.

“The 1964 survey is really widely cited within freshwater mussel literature as one of the earliest properly quantitative surveys of mussel populations, quantifying important metrics like population density, growth rate and annual productivity, and showing that mussels contributed over 90% of benthic biomass (species living in the riverbed sediments),” said Ollard via email.

However, Ollard added that because she and her colleagues knew that many species such as mussels declined by 2020, they returned to the site near Reading, England for a new survey.

According to the scientists’ study published Sunday in the Journal of Animal Ecology, the mussel population declined approximately 95% by 2020.

Also, the scientists found that the mussels were smaller for their age than predicted and that the remnants of the depressed river mussel, Pseudanodonta complanata, were the many empty shells, which the scientists considered worrying on two accounts.

The first is that the depressed river mussel is one of the most endangered mussels in the U.K., and the study described them as "completely gone" from the river. The second is that the scientists consider mussels important in freshwater ecosystems in part because other aquatic species live in mussel shells. Additionally, Ollard says that mussels filter the water, remove algae, and are key in the study of ecosystem health due to their exposure to everything in the water.

As for the mussels' decline, Ollard provides one theory that legislation in England reduced raw sewage in the river, which meant less food for the mussels.

“In the 1960s raw sewage was frequently released into the river, raising concentrations of key nutrients like nitrogen and phosphorus. This would have caused increased algal growth, providing more food for mussels,” wrote Ollard. “Since then, legislation to control sewage outflow has resulted in lower nutrient concentrations in the river (although nutrient pollution remains a problem), reducing algae and food for mussels.”

However, Ollard cautions that she and her colleagues do not believe that nutrient reduction was a driving force for the reduction of mussels in the river. According to Ollard, the arrival of invasive species poses a likelier threat.

The researchers realized that while both the zebra mussel, Dreissena polymorpha, and the Asian clam, Corbicula fluminea, did not appear in the original 1964 survey, they appeared in high numbers in the 2020 survey. The researchers theorized that boats sailing up the Thames carried the invasive species, which then killed native mussels in the 50-year interval between the surveys. However, the researchers remain unsure and urge further research, and offer the additional theories that changes in the land use along the river and changes in the fish population contributed to the mussels' population decline.

And while this survey focused on one river in England, the scientists say that mussels around the world are threatened, with Ollard hoping that this study shows the need for regular population surveys of key species.

“We need to pay attention to these underappreciated species and not take them for granted, because as this study shows they might well be seriously threatened, and it’s crucial that we know about these threats and declines so that we can take action before it’s too late,” wrote Ollard. “In addition, we need better legislative protection for freshwater ecosystems, to conserve mussels and all the other unique species inhabiting them.”

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