This blog post is provided by Isobel Ollard and tells the #StoryBehindthePaper for the paper ‘Declines in freshwater mussel density, size and productivity in the River Thames over the past half century‘, which was recently published in Journal of Animal Ecology. The authors replicated a survey of freshwater mussels in the River Thames from the 1960s and mussel populations had declined to less than 10% of the previous density and explore possible explanations.

Rivers are some of the most precious – and fragile – ecosystems in our changing world. But many of the changes occurring beneath their surfaces are going largely unnoticed. This is especially true for species making their home on or below the benthic sediment of the riverbed. Freshwater mussels, in the family Unionida, are one such taxon. Globally widespread and yet often overlooked, these animals provide key ecosystem services. As filter feeders, mussels remove algae and organic particles, helping to keep water clear and suppress algal blooms. Through biodeposition they increase sediment nutrients and support detritus-feeders. And their shells provide physical habitat and protection for bottom-dwelling invertebrates.

But mussels are under threat: 40% of all species are classed as ‘threatened’ or ‘near threatened’ by the IUCN Red List. Declines have been linked to habitat degradation, such as dam-building and riverbed dredging; pollution with nutrients and toxins; drought; the impacts of invasive species, including invasive mussels; and declining fish populations, which are essential to mussel recruitment since they act as hosts for parasitic mussel larvae.

To track these declines and plan effective conservation actions, we need to build a picture of what mussel populations looked like in the past, and how they are changing. Referring to historical information to reconstruct changes is important to avoid ‘shifting baseline syndrome’, the phenomenon whereby the declining biodiversity experienced by successive generations mean the collective concept of a ‘baseline’ healthy ecosystem is progressively lowered. For freshwater mussels, one of the earliest fully quantitative surveys of freshwater mussels was carried out in 1963-64 in the River Thames, UK by Christina Negus, a graduate student at the University of Reading. The study was key in demonstrating the huge contribution that mussels make to freshwater benthic biomass (over 90% at the study site) and shedding light on aspects of mussel growth and population dynamics.

In 2020, we revisited this site to re-sample mussel populations and assess what – if anything – had changed in the half century since Negus’ study. We were able to talk to Christina Negus herself about her study and the methods she used, to make sure we replicated them as closely as possible.  After re-sampling we found that mussel populations at the site had declined dramatically, to less than 10% of the density present in 1964. One species, the depressed river mussel, which is listed as globally vulnerable by the IUCN, appeared to have disappeared from the site entirely, while the densities of two others, the duck mussel and painter’s mussel, had also decreased severely.  Additionally, two invasive species, the zebra mussel and Asian clam, had colonised the site. We also calculated the growth rates of individual mussels and found that compared to 1964, mussels today are growing more slowly and to smaller sizes. When combined with species loss and density reductions, this means that mussel populations have an annual biomass production of just 7.5% of the level measured by Negus in 1964.

So what is causing this dramatic loss of mussel biomass in the Thames? The presence of invasive mussels could be one answer. The zebra mussel, which is widely invasive across Europe and North America, has been associated with sometimes extreme declines in native mussel populations. They cause biofouling by growing on the shells of the native species, competing for algae and food particles and making it difficult for the mussels to open their valves for feeding. In the Thames, this might have contributed to both declining densities and lower growth rates in surviving individuals.

We also looked at how concentrations of key nutrients (nitrates and phosphates) have changed over the period, using data recorded by the UK Environment Agency. We found that the level of both nutrients declined significantly, with a particularly strong reduction in phosphate. This is probably due to stricter controls on sewage outflow which have been gradually introduced to limit nutrient pollution entering waterways. While the ecological effects of this reduction are overwhelmingly positive – reducing algal blooms and the low-oxygen conditions that develop as a result – the implications for mussels might be more nuanced. With lower nutrient concentrations stimulating less algal growth, there may be less food for mussels, which could translate to slower growth and potentially smaller population sizes. In this light, it is possible that the higher growth rates recorded by Negus in 1964 were elevated above ‘natural’ baseline levels due to anthropogenic nutrient enrichment, and that the lower growth rates and biomass production we found are a reversion towards pre-anthropogenic levels. However, it is unlikely that the significant declines in population density can be explained solely by this loss of nutrients, and other threats, including invasives, are almost certainly also contributing.

These findings reflect reported losses and decline in mussels globally. Alarmingly, we found that declines occurred not just in species already identified as threatened, but also supposedly common species, which in the UK are not currently protected or even routinely monitored. If declines of similar scale are occurring more widely, this could point to a catastrophic loss of mussel biomass and the ecosystem services they provide. Better monitoring, including of apparently non-threatened species, expansion of the use of historical information to understand population trajectories, and interventions – both targeted at mussel populations, and broader protections for freshwater ecosystems – will all be required to safeguard this important taxon for the future.  To safeguard for the future healthy, resilient mussel populations supporting diverse freshwaters that function for people and nature, the time to act is now.

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