Revisiting 1964 and a unique river mussel study

In 1964 an oft-cited study was carried out on the freshwater mussel populations in the River Thames near Reading in Berkshire. Over half a century later, zoologist Isobel Ollard revisited to the site to gauge what had happened in the intervening period

DOI.ORG/10.58214/IOUC0203

Profile: Isobel Ollard

Isobel Ollard is a PhD student in the Aquatic Ecology Group at the University of Cambridge.

She studies the ecology and conservation of freshwater mussels, focusing on tracking population changes over time, and the interactions between mussels and other freshwater invertebrates.

Top image: © Eddie Starck (CC BY 2.0) https://creativecommons.org/licenses/by/2.0/legalcode

It’s not often that an historic study is so unusual and unique in its scope that today’s scientists can hope to carry out an identical study and be able to compare the two. But that is what has happened on a short stretch of the upper River Thames near the Berkshire town of Reading.

University of Cambridge zoologist Isobel Ollard chose to recreate the research undertaken by University of Reading PhD student Christine Negus in 1964, which centred on the health, or otherwise, of the native freshwater mussels in that particular stretch of the Thames.

What was uncovered was perhaps depressingly predictable; native freshwater mussel populations are a fraction compared to what was found in Negus’ time.

Aether spoke to Ollard to find out why and whether there is anything that can be done to reverse the decline in native mussel populations going forwards.

Aether: Can you tell me a little bit of the history behind the 1964 study and why you decided to use that as part of your own research?

IO: This study was done by a student at the University of Reading, Christina Negus, as part of her postgraduate studies. It was one of the first really quantitative surveys of freshwater mussel populations anywhere. It quantified things like the proportion of benthic biomass, so the biomass of everything in the riverbed that was contributed to by the freshwater mussels, and Christina showed that that was over 90%.

She also looked at the densities of different species in the river and their growth rates each year. For example, she kept a few mussels back in the lab and she was one of the first to show we get these annual rings which form on their shell. They are like dark bands on the outside of the shell and, a little bit like tree rings, they are formed annually, so they can be used as a way to estimate the growth of the mussel and how much it grows each year.

Her research is widely cited, even today, as one of the first properly quantitative surveys of mussel populations and as evidence of how important a contribution mussels make to riverbed ecology.

Aether: Why are river mussels so important?

IO: As Christina showed, they’re really abundant, but they also have a few really important functions. We call them ‘ecosystem engineers’ because of the way they affect the wider ecosystem.

First of all, mussels are filter feeders. They take in water through their siphons and they extract out algae and other small organic particles which they feed on. A single adult mussel can filter over 40 litres of water a day.

They have a really big impact; when you think across a whole riverbed with potentially thousands of mussels, that’s a huge amount of water filtration.

That helps to do things like reduce the amount of algae in the water column, which in turn allows better light penetration through to the riverbed and prevents the build-up of harmful algal blooms. They have other impacts as well; because of their filtration, they excrete nutritious, for some organisms at least, faeces into the riverbed, which acts as a kind of nutrient transfer mechanism from the water column into the riverbed.

Their shells also provide a really important habitat for species which either grow on the shells like some algae and small invertebrates, or they provide a shelter or a refuge from predation and from currents in the river. The rivers which have a higher density of mussels also tend to have higher biodiversity in terms of macroinvertebrates.

Assorted mussels found during the survey © University of Cambridge.

Aether: Mussel populations are worryingly lower in the River Thames than in 1964, but you suggest that might be down to the river being much cleaner today because a polluted river made for a better environment for mussels. Is a less polluted river a bad thing for mussels?

IO: There are a couple of things going on here. We found that both mussel population and mussel sizes are declining, so there are far fewer mussels today than there were in the sixties, but we also found that the size of each individual mussel has got smaller.

What we think is likely is that that size decrease is connected to a reduction in nutrient concentrations in the river because fewer nutrients means less algal growth and therefore less food for mussels. We think that might mean they’re growing smaller.

But I think it’s very unlikely that that alone is what’s caused this massive drop off in the numbers of mussels. For example, we know that it is much lower than the potential densities we find in pristine rivers where there isn’t any anthropogenic nutrient enrichment. So, I don’t believe the loss of anthropogenic nutrients is what has caused the decline in numbers of mussels.

Aether: How serious do you consider the invasive species threat to Britain’s indigenous river mussels?

IO: Very seriously. The invasive mussels, especially zebra mussels, are really good invasives: they reproduce very quickly; and they can grow almost anywhere. Unlike native species of mussel, which have to burrow into the riverbed sediments, they are able to grow on hard surfaces as well.

That means they can grow on rocks or concrete, and you often find them lining the sides of reservoirs, for example.We find we’re actually converting freshwater habitats into ones which favour invasives over natives. They can also grow physically on the shells of native mussels, and you can sometimes find really high fouling density, perhaps more than 20 zebra mussels growing on a single native mussel. That eventually means that they can’t physically open their shells to filter and to breathe. They also compete for food with native mussels, and they can grow at really high densities.

That combination of competition for food and space and actually biofouling the shells of the native mussels is a massive problem for native species in the UK.

Aether: If a stretch of river used to have native mussels in it, but they have been completely replaced by invasives, how does that impact on the ecosystem of the river?

IO: Some ecologists view it that although we’ve lost the native mussels, we’ve gained invasive mussels which play some of the same roles in the ecosystem such as water filtration. The view is, perhaps one mussel is much like another and there’s no impact on the wider ecosystem. We don’t have enough data to say that for sure at the moment, but we know that there are some differences. For example, some studies have shown that invasive zebra mussels are selective of different algal species than native species, and that they might actually promote the growth of toxic algal species such as cyanobacteria.

The different feeding patterns alter the balance of algal species in the ecosystem which could favour the development of more toxic algal blooms.

There’s also an argument to be made that we should try to conserve native species for their own intrinsic value. I think it’s very dangerous to say that as it looks like these species are doing a similar thing, so it’s probably okay if we are losing native species of mussels and they are all being replaced by zebra mussels.

That is something that’s happening around the world, so in the Great Lakes in the USA for example. It is happening on a really wide scale. That kind of overall degradation of biodiversity is something that we should be worried about.

Swan mussel (Anodonta cygnaea) found during the survey © University of Cambridge.

Aether: Where have the zebra mussels come from?

IO: They’re native to western Russia, along the border between Russia and Europe. We think they were probably originally translocated by shipping. The larvae of these mussels and the mussels themselves can survive pretty well for quite a long time in not particularly good conditions. They are also really good at reproducing, and they produce an enormous number of larvae. I think the conditions have created a kind of perfect storm for them to do really well.

Aether: What do you feel needs to be done to improve the populations of native mussels in the Thames and elsewhere?

IO: That is difficult to answer. Unfortunately, because it’s not the easiest thing to do, I think it’s about protecting the health of the river ecosystem more widely.

If you have a healthy, intact river, the mussels will do well. That means reducing pollution, not only of nutrients, but also of things like pharmaceuticals and heavy metals. We are only just starting to understand the kind of impacts that they can have.

Another is improving connectivity so that if you have a dam that blocks off the river then that is reducing the amount of nutrient flow through the river. Leaf litter that has fallen upstream is no longer getting downstream.

Dams can have a real impact on mussels because they have this unique life history strategy where to reproduce, they produce larvae called glochidia, which are effectively just a little set of jaws, which are released into the water where they have to latch onto the gills of a fish. They stay there for a few weeks effectively being a parasite as the fish swims upstream. At some point the larvae will drop off, land in the sediment as a juvenile mussel, and that is where it will spend the rest of its life.

Dams which interrupt fish migration also interrupt the ability of the mussels to use those fish as hosts for their larvae and therefore to get upstream as well. It’s just another example of how wider ecosystem health can also affect mussel population health.

Aether: Going forwards, are you going to look at mussel populations in other UK rivers or would you prefer immediate conservation measures to act on what you already know?

IO: Can I say both? We definitely need more research on other rivers in the

UK as well as elsewhere, but the UK is particularly bad for monitoring freshwater mussel populations. Many countries in Europe have obligatory mussel population monitoring as part of their environment programmes, but we don’t really have that here. As a result, we don’t have a good idea of how widespread these declines are across the UK, so replicating this kind of research elsewhere is going to be really important for building up a picture.

That, in turn, will help us to plan and direct conservation actions more effectively towards, perhaps, regions where mussels are most threatened or if we can work out which particular stresses might be constant across rivers where mussels are showing particular declines.