Episode 9: Oysters in the Solent - cleaning up our waters

Dr Joanne Preston talks about how she's been reintroducing native oysters into the Solent

18 min listen

External Audio


Dr Joanne Preston tells us how she's been reintroducing native oysters to the Solent with pioneering tech. The biodiversity and water quality benefits of these humble creatures can be astounding, but after years of over-fishing, is it possible to reintroduce them in the Solent and supercharge our waters once again?

Discover how she’s built special ‘cages’ to accommodate the new population and help overcome challenges to their settling and reproduction.

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If you push an ecosystem so far, if you lose too much biodiversity, then it loses resilience and it really loses resilience to future change.

Dr Joanne Preston, Principal Lecturer

Episode transcript:

John Worsey: Thanks for downloading this podcast from the University of Portsmouth. I'm John Worsey, a writer and in Life Solved, we're asking the big questions about our world, from politics to technology, our bodies and our environments. Today, we'll hear how a team of scientists at Portsmouth has been experimenting with a native species that's got the potential to clean up our waters, conserve our marine environments and protect our coastlines.

Jo Preston: The impact they can have on water quality is phenomenal if they're in high enough numbers. And that can control the nutrient status of waters or the nitrogen and phosphate runoff and things and after sewage.

John Worsey: But human activities nearly wiped out our native oysters. Is it too late to save them?

Jo Preston: If you push an ecosystem so far, if you lose too much biodiversity, then it loses resilience and it loses resilience to future change.

John Worsey: One group of researchers at Portsmouth thinks there may yet be hope. Emma Fields spoke to Dr Joanne Preston.

Jo Preston: I look at marine organisms, mostly in coastal environments, in temperate environments and not tropical. I do a lot of work using DNA. Looking at the DNA sequences of lots of different organisms and see who's related to who. And then you can basically use that to look back in time and see how things have evolved. And then you can understand - you have these patterns of evolution - and then you can understand the processes of evolution. Then you can start to see how marine organisms and evolved over time and how diversity is generated. I work with organisms that seem really dull and boring, like sponges or oysters and things. And then when I start investigating what they do and some of the sort of facts behind it, I'm like, wow, I never knew that.

John Worsey: The focus of a lot of Jo's recent work has centred around the humble oyster. Why? Because these little creatures have a huge impact on our coastal and marine environments.

Jo Preston: They're bioremediators, they'll sort of hoover up clean, filtered water. So all those benefits that come with that, you know, I mean, they're sort of eutrophic green pea soup we get out there it's always going to be a nutrient-rich system because of mudflats and things. But there didn't used to be as much money as there is now. There's far more mud than they used to be.Less salt marsh, less seagrasses, less oyster beds. And if they're removing all the nutrients the algae can't grow, you get clearer water. That means the light penetrates the seagrasses. The seagrasses will grow better, and you get some amazing gullies and sea walls off Ireland, off the Welsh Pembrokeshire coast. Also, one of the things that I'm very much interested in the moment is restoration of the native oyster. And one of the things about this species, or oysters particularly, is they're referred to as biogenic or ecosystem engineers, which means they sort of create a whole ecosystem. Oysters themselves historically would have formed these oyster reefs that don't exist anymore in temperate waters. They've been completely wiped out. There used to be this massive bed in the North Sea, around 40 metres deep, that was just completely just fished out.

John Worsey: The overfishing of oysters and damage to their natural environments has had a huge impact on the prevalence of these creatures in past decades. Jo and her students have been experimenting in local waters.

Jo Preston: 85 per cent of oyster reefs are extinct. So there used to be, the global coverage has been diminished by 85 per cent. Emma Fields: And that's the impact of humans overfishing?

Jo Preston: Over extraction and pressure development. And of the ones that remain, 15 per cent remains, most of them are in poor condition. It's like the temperate equivalent of a coral reef. Several waves of disease wiped them out, which is probably related to the over-extraction in the first place. But oysters themselves create this three-dimensional hard substrate that supports a huge amount of biodiversity because it becomes a habitat for other things to live on. And well, one of the big drivers is the loss of biodiversity that has occurred over the last sort of one hundred and fifty years due to anthropogenic disturbances, impacts, and there's been many mass extinctions over the history of our planet. But this one is driven by humanity. And if you push an ecosystem so far, if you lose too much biodiversity, then it loses resilience and it loses resilience to future change. It has less tools in its box, basically. Less capable to adapt to, say, climate change or future pressures, be it pollution. We did 120 grabs or more in Portsmouth, Langstone and Chichester Harbour, and I think we found three oysters in all of those. The highest density of perpetual I think was eight thousand per metres squared. There's ones that look like little slippers. Now that's created a fornicator and that's the invasive mara slipper limpet. That came over with oysters in the 1920s into Essex and has just sort of colonised. And the seabed now in Langstone is entirely dominated by pages fornicator. Which is one of the problems because the oyster larvae don't like to settle on [00:05:45]crepidula fornicata. [0.0s] So there's been an ecological shift - phase shift. Probably combined with a bit of poor quality. Basically, the oysters no longer really on the seabed, and so an ecological niche became available. And the slipper limpet has no native predators. It's an invasive species and often invasive species that has no real natural predators.

John Worsey: Jo studied restoration work in America and around the world, where reef restoration shown multiple other benefits to coastlines and environments. She wondered if it was possible with our own native species, given how badly they'd been diminished. There was only one way to find out.

Jo Preston: We're trying to conserve a species back to an impoverished state because we've forgotten what the more pristine state was. And so what some people are doing is going through like archaeological evidence and middens and old transcripts to try and finally get a handle on what the ecosystem used to look like back in the late 1700s, early 1800s before we over-extracted so much to try and sort of get back to-- because otherwise, if we try to restore it back to an impoverished state, then it's not going to be resilient enough in terms of density and things like that. So in the states, I'd combined a lot of work doing oyster restoration at the shoreline of salt marsh, which is a wave break and promotes sedimentation on the other side, which means that salt marshes, which are being eroded due to wave motion are no longer and the sedimentation dynamics stop, and then the salt marsh starts encroaching, and that's what we want to try in the Hamble to see if we can do it with this species.

John Worsey: Jo and her PhD student were planning an experiment to see if placing artificial panels around the harbour would simulate the environment of a seabed or benthic zone and allow a diverse life to bloom here once again. Around the same time, Ben Ainslie's sailing team were looking to collaborate on building something sustainable as part of their new headquarters in the Solent. They got talking.

Jo Preston: I said, well, why don't you try to do something under the under the water? And we were going to try and stick these sort of panels around the harbour to increase biodiversity. And then the Blue Marine Foundation doing the Oyster Project was interested as well. So then we just joined forces. And so we decided, well, what if we put them in cages suspended for marinas, then we can monitor them. There could be fish, they're less likely to get disease because it's not on the seabed, they're less likely to be predated on by crabs and whelks. What we're doing really is we're sort of, basically putting an oyster reef in vertically so it's completely is sort of-- it's an artificial construct that we're trying to recreate that oyster reef. And we didn't know whether they'd like being suspended, they're benthic organisms, but they do it. They've done it with fisheries. It's like a Pringle tube style stack that you slot oysters in, and it means they're sort of orientated for maximum feeding and various other things. And we tied four of these together and put three of those together and we got a caged-- metal cage built around it. And that was our, that was prototype three I think. The first time, we just had nets tied to the frame, the metal frames. And so then we've rolled that out to six marinas across the whole of the Solent.

John Worsey: As soon as Jo's initial experiments began to fair well, a marina group called MDL got behind the project, helping to fund a refined design. The potential for multiplying these mini-ecosystem boosters is really exciting.

Jo Preston: You can get little dabits you just put on the side of the marina. What I think we just did them half because what you want to do is roll out to marinas across the UK.

John Worsey: Cleaning the oyster cages is a grim but vital task to the survival of the oysters. And it was through this process that the experiment led to another exciting realisation. The oyster cages weren't just a booming sight for oyster activity, but for surrounding organisms, too.

Jo Preston: My undergraduates started looking at the biodiversity associated with oysters in the cage system that we've developed. And he found 95 different species associated with oysters. When we have to clean the cages, bring them up, they're just completely foul. There's just so many things growing all around it. And so we have to sort of manage the cages and get the water flow through and so they can eat all that sort of phytoplankton, the algae and everything in there. But that's when we noticed like how biodiverse and it was, and it's very slightly different to the biodiversity on the seabed, but not that much, and it changes quite a lot from different sites, the type of species are found there. It acts as a bit like an artificial reef but suspended. The biogenic habitat. So it's basically just a habitat of biological origin. As they sort of on the sea bed oyster beds they die. They leave shells, they're very gregarious, they like to settle on each other's shells. So oyster larvae love oyster shells more than anything. So then they create this three dimensional structure with lots of nooks and crannies and the calcareous sort of substrate is a settlement substrate for like policates and tunacates and various other things -- they're worms, they're sort of segmented worms that live in little calcareous tubes. Tunacates are sea squirts. There are things that are ascidians and sponges, and then you get some algae growing on it and then it becomes a refuge for juvenile fish. You get lots of crustaceans, shrimp things like grazing on it and then bigger fish. So it becomes like what kinds of nursery ground for smaller fish to feed on the little bits of sponges and crustaceans and be protected before they go into the ocean. It's brilliant, and it sort of still we're sort of working out what's the later model, the turnover rate of the cages, how many-- because they only live to five or six years in the wild, they might live slightly longer in the cages. So that's basically the system. And now we've started off, we've tested the different densities to see which work, we're testing if they reproduce. Testing when they reproduce.

John Worsey: The question of reproduction hung in the air. Without a happy breeding oyster community, the cage experiment would be a failure. But the team soon realised that the artificial environment was setting the mood for some unexpected results.

Jo Preston: Now, oysters protandrous hermaphrodites, which means they alter sex sequentially. And so they-- this is my one joke I ever tell if I speak-- they start off male and they improve. [LAUGHTER]. Two to three years old, they become sexually mature and they become female. But then they can swap back to male and female throughout the growing season. But what is thought is that the ratio is density-dependent. So there's no point investing in Bill Napoli's ovaries that there are no males around. One of the problems is that such a low density on the sea bed, so they're not very reproductively viable, but put them in a cage, there's like we had sort of 120 oysters next to each other, so they were having a party. So then we monitored how many of them are reproducing and how fecund they are, how fertile they are. We still got lots of pots of gonads and various things to analyse. But they're reproducing really well in the cages, which is again a surprise because there's greater stresses of temperature variation of things at the surface than there is the bottom.

John Worsey: After finding that invasive slipper limpet or crepidula shells were dominating the seabed, Jo and the team have been putting out roof tiles to see if they can mimic a natural shell-like place for the oyster larvae to call home. There's hope that this will tip the balance towards a more stable oyster population and in turn support other diverse organisms in these environments.

Jo Preston: We put different settlement plates out to look for oyster settlement by the cages, and we put plastic concrete crepidula shells, old oyster shells and new oyster shells, and the crepidula settlement was slightly better than plastic but worse than concrete and sort of, I think, you know, 10 times more settlements found on the oyster shells. So not only have they been out-competed, if you think about those larvae, they're swimming around trying a find their nice little oyster shell, and they keep on touching down and finding crepidula, and they will sort of probably delay their settlement metamorphosis. That probably has an implication on their sort of future fitness. And so what we've been doing is putting settlement plates out in locations that have cages and don't have cages using a model from hydranite model of where the particle should and shouldn't go to see if there's any settlement from -- well, we don't know it's from the cages per se, but a settlement in proximity to the cages. One female oyster in her prime, like every time she spawns, she'll produce like a million larvae or more. So obviously not many of those survive. You know, it's between one and five per cent. But even so, we have got now a moment, we did start off with a few thousand oysters in cages. We went up to 10,000. Now we just put more out and theres 20,000 oysters in cages around the Solent now.

John Worsey: So the challenge isn't only about creating a good environment for oysters to live and reproduce in for the short term, but also in managing the wider system so the larvae have a good spot or substrate to settle into. Jo is passionate about her vision of restoring local oyster populations in the Solent, and the dramatic impact of reviving these benthic-pelagic, that's living near the bottom of the sea, creatures.

Jo Preston: In terms of the sort of biology, ecology and marine success would be to see a restored oyster population that is self-sustaining and that might take more than five years. It probably will. But an oyster population that is well enough protected and managed to be self-sustaining, self recruiting, and it could be transformational. What we see out there is not how it looked 100 years ago, 150 years ago, there were no slipper limpets at all out there. So the big piles of capidula on the beach didn't exist. The seabed wouldn't have been the fine silty mud. It would have been encrusted with oysters and all the biodiversity associated with that. Going back to 150 years, there would have been far more salt marsh, there would have been more fish, greater biodiversity and the water quality would have been far clearer because one of the things oysters do is they're filter feeders. All they do is filter the seawater, consuming the algae and bits of organic matter therein. And so they have a big role in benthic pelagic coupling, bringing things in the water column and taking into the benthos. And the impact they can have on water quality is phenomenal if they're in high enough numbers. And that can control the nutrient status of waters or the nitrogen and sort of phosphates that come from runoff and things and out of sewage. And it's those nitrates and phosphates that cause the algal blooms, the big green algal blooms and that causes reduction in oxygen in the water. One oyster can filter 200 litres of seawater a day. There's some debate that oysters, actually, whether up to a certain density they're a carbon sink. But after a certain point, it might become a carbon source as well. But you know, for human impact, basically, you'll get a pretty, slightly pretty marine environment. Better for recreational fishing, better water quality, better biodiversity, slightly less green, slimy mud,

John Worsey: Jo's now turning her attention to sponges, another symbiotic hero of the sea and work is ongoing with the oysters in the Solent. Let's hope in the years to come, we can all enjoy the positive impact along the coast and in our harbours of a revived and burgeoning population and the diverse ecosystems they'll create. You can follow our research at port.ac.uk/research. Our new magazine Solve follows University of Portsmouth research when it's put into practice, it's full of news and stories and our world-leading advances and the changes these are making to lives and futures across the world. You can read more from Joanne's work with our native oysters in the very first issue. It's available at port.ac.uk/solve. Next time on Life Solved, how easy is it to change the law?

Juliet Brook: Sooner or later they are going to also think about reforming an act that is 180 years old. Then you get the case law coming in.

John Worsey: You can share this podcast using the hashtag life solved. Or maybe just share the big idea with a friend. If you subscribe in your podcast app, you'll also get each episode of life solved automatically. I'm John Worsey, and we'll be back with another story next time.

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