A dramatic slump in mussel populations found in the River Thames highlights a worrying deterioration in its ecosystem over the last six decades, say scientists in the United Kingdom.

A survey, carried out in a section of the river west of London near the town of Reading, found that native mussel numbers had gone down by almost 95 percent since the 1960s.

The Guardian newspaper reported that the study found the population of a variety called the painter's mussel had decreased to 3.2 percent of 1964 levels, the depressed mussel has disappeared entirely, and other species were found to be smaller for their age.

The population decline is alarming, said David Aldridge, professor at the University of Cambridge department of zoology, and lead author of a report on the survey published in the Journal of Animal Ecology.

"While this might seem like a rather parochial little study of a single site in a single river in the UK, it actually provides an important warning signal about the world's freshwaters," he said.

The study noted the importance of adult mussels to a river's ecosystem as they are able to filter up to 40 liters of water a day, a natural process that helps keep rivers clean by removing huge amounts of algae.

"Mussels are a great indicator of the health of the river ecosystem" said Isobel Ollard, from the University of Cambridge and a co-author of the paper.

"Such a drastic decline in mussel biomass is likely to be an indicator of a deteriorating environment," she said. "It is also likely to have a knock-on effect for other species, reducing the overall biodiversity."

The study set out to replicate a survey conducted in 1964 that provided evidence of the major contribution mussels make to river ecosystems.

The researchers say the population decline may have been caused by invasive species that might have fallen off boats as they traveled up the Thames.

Large numbers of the nonnative zebra mussel and Asian clam, which were absent from the original 1964 study, were noted in the latest research.

Scientists pointed to the zebra mussel as a possible key culprit for causing the population decline as it is known to attach and grow on native species, smothering them to death.

Shifts in land use along the river or changes in the fish populations mussels rely on were other factors considered as possible causes for the extreme fall in numbers.

Tighter regulation of sewage treatment since the 1960s is thought to be the reason for reduced growth rates, as this would lead to lower levels of algae and therefore a reduction in nutrients available to the mussels.