Where the River Meets the Sea: Everything you wanted to know about estuaries but were too afraid to ask

For a long time, freshwater experts thought of estuaries as just "the end of the river," while marine biologists dismissed them as "the bit where the sea comes in." But today, scientists realize that estuaries are incredibly complex, dynamic ecosystems in their own right. 

In this episode of the Nature Recovery Podcast, Stephen is joined by world-renowned estuarine and coastal science expert, Professor Mike Elliott. They explore the fascinating, constantly fluctuating world of estuaries, the vital "ecotones" where rivers meet the sea.

Professor Elliott explains the "triple whammy" of pressures facing global coastlines, the difference between contamination and pollution, and introduces the concept of the "Emerald Economy." He also breaks down the DAPSI(W)R(M) framework for solving complex environmental challenges, proving that when we relieve the pressures on these dynamic systems, nature can, and does, bounce back.

Key Topics Covered:

• What actually makes an estuary an ecosystem in its own right?
• The "Estuarine Quality Paradox" and how organisms survive extreme environmental stress.
• Understanding the "Emerald Economy": Ecosystem services vs. Societal goods and benefits.
• The "Triple Whammy" threatening our coasts: industrialization, resource depletion, and climate change.
• Incredible recovery stories: How the dead Thames and Clyde estuaries were brought back to life.
• The DAPSI(W)R(M) framework: A roadmap for balancing human needs with natural systems.

Resources:

• Learn more about Professor Mike Elliott's work at the University of Hull and IECS Limited.

The Leverhulme Centre for Nature Recovery is interested in promoting a wide variety of views and opinions on nature recovery from researchers and practitioners.

The views, opinions and positions expressed within this podcast are those of the speakers alone, they do not purport to reflect the opinions or views of the Leverhulme Centre for Nature Recovery, or its researchers.

The work of the Leverhulme Centre for Nature Recovery is made possible thanks to the support of the Leverhulme Trust.

Transcript

Stephen (Leverhulme Centre for Nature Recovery): So welcome to the Nature Recovery Podcast, and my guest today is Professor Mike Elliott. Mike is a leading expert in estuarine and coastal science, with a long-standing career dedicated to understanding and protecting these vital environments. Currently based at the University of Hull, his work spans ecology, environmental management, and policy, with a particular focus on how human activities shape estuarine systems. Mike is widely respected for bridging science and practice, and helping to translate research into real-world solutions. If you Google who are the top experts in the world on estuaries, you will find Mike's name very high up there. So thank you for your time, and welcome to the podcast, Mike!

Mike Elliott: Thanks, Stephen, it's great to be with you. Just very quickly, I should mention I'm an emeritus professor at the University of Hull, and also director of a research company called IECS Limited. But we can touch on these later.

Stephen: Fantastic. So again, bridging both worlds, which is like an estuary. As someone that didn't know that much about estuaries coming in here, maybe we can start with just explaining really what they are. If we think of estuaries as being a bit more than just a place on a map where the river meets the sea, or inland water hits coastal water. If we started to think of it as a living system, could you give us an overview of what the main parts are, the roles each part plays, and what's really happening within an estuarine system?

Mike Elliott: That's quite an interesting observation. For many years, they were always regarded by the freshwater people as the end of a river, and by the marine people as the bit where the sea comes in, and that's about it. It's only probably these last three or four decades that we've realized they are an environment and ecosystem in their own right. And the reason they're in their own right is because of something you said, Stephen: it's that connectivity. They function and have a structure because of the connectivity between the sea, the land, and the inland catchments.

So, their main features are their connectivity. They're probably one of the most varying environments we've got. If you think about it, if someone stands in an estuary, at one time it's low water, which is mostly river water coming down, and a few hours later, the sea comes in, bringing fully marine conditions. The organisms that live there have to cope with that high variability. Because of that variability, in the jargon, we talk about a "stress subsidy" approach or continuum. That is, if you're not suitable to live in those conditions—if you can't put up with the salinity changes, for example—it's stressful. But if you are adapted to living in those stressful conditions, it's a subsidy. That's why we don't have massive amounts of species there, but we do have large numbers of each species. We have dominant organisms that live there. Once you get the invertebrates living there, you get the birds and fish feeding on them.

One of the other main features of estuaries is that these are where most of our major cities around the world are situated. The stresses and pressures on them are what we call a "triple whammy." The triple whammy for these areas is: first, they are increasingly industrialized and urbanized (think about London); second, there is an increasing depletion of resources, like removing fish, shellfish, water, and sediments; and third, they have a decreasing resistance and resilience to climate change. Climate change is going to affect them more than anything. Those three things are putting more pressures on them. So you've got all this amazing wildlife, and at the same time, all this human stuff going on.

Stephen: Yeah, I think you've probably answered my next question a bit, which was just thinking about what's happening there. You've got river inflow bringing sediment, tides bringing salt, and that's all interacting to shape the energy of an estuary and the life it can support. Are we mainly talking about shellfish, fish that are coming up, and birds that wade or swim? Are those the main trophic levels, or are there other mammal or keystone species that thrive in estuaries?

Mike Elliott: Well, Stephen, we work on the basis that the physics and chemistry set up the system—what the waters are like, the tides, the sediments, and so on. Once that's there, the organisms come in and colonize it, as long as they're able to cope with the salinity or other factors. Once the organisms are there, they start interfering with each other through predator-prey relations, competition, and so on. Interestingly, in these environments, the organisms themselves can start modifying the physics and chemistry.

I'll give you a couple of quick examples. We have a lot of fish that move between freshwaters and marine areas, like salmon. If we get a huge number of juvenile salmon coming from the catchment into the estuary before going out to sea to grow, those fish can exert an oxygen demand—they can take the oxygen out of the system. Another example is the worms and shellfish in the muds. They're constantly turning over the sediment, renewing it, and changing nutrient levels.

So within that, you've got this cycle: the environment creates the biology, the biology modifies itself, and then the biology influences the environment. That's different from what we thought 50 or 60 years ago, when we focused purely on the environment affecting the biology. Good examples are ecosystem engineers, like seagrass beds or salt marshes—they create the environment, and once they've done that, other things come in.

Stephen: Amazing. I hadn't thought about oxygen as a key thing that can vary in estuaries between river and sea. In ecology, we often separate marine and terrestrial, and in the public consciousness, we think of land and sea separately. I find estuaries fascinating because they sit in this in-between space. You've called this the "emerald economy." Can you explain what you mean by that, where the term came from, and why we should focus more on it?

Mike Elliott: Yeah, we started discussing this jointly with Polish and Lithuanian scientists. We've always talked about the "green economy" (the land-based system) and the "blue economy" (the sea). But we realized we've got this bit in between where we get benefits that rely on the fact that both the land and the sea are operating on the system.

Some of your listeners will have come across the term "ecosystem services"—meaning the environment is doing something, like producing fish. But what we've been working on is how you turn that service into a societal good and benefit. The fact that an estuary can support fish means eventually, we might want to use those fish. We have to learn to catch the fish, prepare it, build restaurants, and build boats.

It helps to split ecosystem services from societal goods and benefits. What are the ecosystem services in an estuary? Producing fish, recycling nutrients, storing pollutants, etc. How do you go from that to societal goods and benefits? That is the emerald economy we are trying to capture. It's not just fish; it's using the system to protect our lands from flooding, or taking out sand and gravel for building. We focus on the emerald economy because it's not just the sea and it's not just the land; it's there because you've got both.

Stephen: I love anything that brings money into nature, like payments for ecosystem services. But I'm always struck by the thought that we don't actually know all the ecosystem services yet—we're constantly discovering new things that nature provides. It's interesting that we can look at the fish and say there's value there, but what if we're not protecting spawning grounds or the connectors?

Mike Elliott: The reason we started splitting ecosystem services from societal goods and benefits is that it's the societal goods and benefits that we can value in monetary or cultural terms. Ecosystem services we value in nature terms. We are looking at valuing nature in socio-cultural and socio-economic terms, but also valuing nature in ecological terms—its own intrinsic value, nothing to do with people.

The problem we have is that to get politicians' or the public's interest, you have to ask, "What's in it for me?" So that valuing is important. But we can value nature for its own benefits. People talk about "cultural ecosystem services," but nature doesn't know culture. Nature doesn't know what a nice view is, or that a blue whale is more important than a worm. We put that culture onto it. But if you look at the estuaries around the world, their main pressures are there because they support major cities and ports.

Stephen: You literally wrote the book on estuaries. Do you view an estuary as a single ecosystem, or is it a connection of micro-habitats like mudflats, salt marshes, channels, lagoons, rivers, and the sea?

Mike Elliott: It is a connection of them all. What we've written about is that it is a system on its own. We published a paper with about eight central paradigms that apply to estuaries all over the world. But at the same time, some listeners will have heard the term "ecotone"—a merging between two systems. Earlier papers said the estuary is one ecotone between the land and the freshwater. Then we realized it's an ecotone between the sea and the estuary, the estuary and the freshwater, the estuary and the banks, the surface and the water column, the water column and the bed sediment. It's all of those ecotones!

That's what makes it special, but it means you've got variability across all of those. That's why you have to ask if organisms can cope with those stresses. We came up with something called the "Estuarine Quality Paradox," which says you get features in an estuary that look exactly like stressed systems anywhere else, but these are natural stresses. Under the stress subsidy idea, if you can put up with the stress, you thrive. If you can't, you get out.

Stephen: That's a good point to think about the increasing stresses—some natural, but some coming from human pressures. I'm very aware of land pressures like agriculture and deforestation, and marine pressures like overfishing, plastic, and river pollution. Talking to you makes me realize that river pollution just flows down and hits the estuary at some point. How are estuaries faring? What are the pressures they're facing, and do they have their own distinct threats and solutions?

Mike Elliott: Throughout human history, we've always put our rubbish into the nearest bit of water, assuming it magically gets taken away. The only problem is, where does it go? The estuary receives all that. We talk about an estuary as both a sink and a source. It receives all the wastes from the catchment—litter, and particularly in countries like the UK, nutrients.

If the body of water can cope with it (what we call the assimilative capacity) and deal with the nutrients or break down organic matter from sewage, then fine. But if it can't cope, we get adverse effects. Nutrients from the catchment cause eutrophication—the system becomes too organically enriched, leading to algal mats, smelly muds, and a loss of oxygen. Eventually, that stuff moves out to sea (the "source" bit).

Litter is a big problem because it doesn't disappear; it just breaks down into smaller particles like microplastics. In places like the Humber estuary, which drains a large part of industrial England, it's been receiving pollution for a long time. We split "pollution" from "contamination." An area is contaminated if it has extra materials in it, but if those materials have a biological effect, we call it pollution. For many years, estuaries like the Thames and the Clyde were totally polluted and had no fish. We exceeded their assimilative capacity. We've cleaned those up now, but we've got all these other things happening—dredging, navigation, fisheries, recreation, energy retrieval, and urbanization.

Stephen: You spoke historically about degraded systems and eutrophication. With rivers, it's hard to clean them up because your neighbor might be polluting further down. Estuaries seem like your last chance to do something about marine pollution before it reaches the ocean. When you see nature recovery, what's happening? Is it all upstream, or are there interventions at the estuary itself? How quickly can they recover?

Mike Elliott: We used to think systems were much more delicate than they are. "Resistant" means they can withstand stress; "resilient" means they can bounce back from it. We now realize that if you stop those pressures, systems often recover quite quickly. In a fast-flowing river, if you stop the sewage, it recovers quickly. In an estuary, materials hang around longer because the water oscillates back and forth with the tides, but once you remove the pressures, the system does recover.

The Thames estuary used to be dead—there were no fish in it. Eventually, the Houses of Parliament did something about it when the smell started affecting them having their G&Ts on the Terrace! For much of the last century, there were no fish in the Thames. In the early 1960s, there were one or two species. Now there are about 120 species, which is what you'd expect. It has recovered, and places like the Delaware, the Clyde, and the Scheldt have seen similar recoveries.

We have to tell countries that are currently busy polluting their estuaries to learn from us. Even if you can put up with the smell, you will eventually lose the emerald economy—you won't be able to fish or bathe in there. The speed of recovery is often related to the variability of the system. A highly variable system recovers quite quickly. In the past, sewage took all the oxygen out of the Clyde, and salmon were coming to the surface to breathe while locals stabbed them with pitchforks. In the Forth estuary, distillery waste did the same thing. Now we've learned from that.

Stephen: You've worked as both a practitioner and an academic. When working with researchers and groups to clean up estuaries, how complex is it? Do we have good data to know what the problems are, and how do you coordinate action across such complex scales?

Mike Elliott: Before I came to the University of Hull, I worked for what is now the Scottish Environment Protection Agency for 13 years at the sharp end. Regulatory bodies aren't allowed to put industries out of business, but they can give licenses with restrictions on what they can put out.

We have a framework called DAPSI(W)R(M).

• D (Drivers): Our basic needs—food, water, enjoyment.
• A (Activities): We carry out activities like fishing or aquaculture.
• P (Pressures): These activities create pressures (mechanisms of change).
• S (State change): The pressures act on the natural system (fish, birds, etc.).
• I(W) (Impacts on Welfare): If we affect the system too much, it affects us (e.g., no fish to catch).
• R(M) (Responses using Management Measures): We do something about it. We create laws, economic incentives, or technologies like sewage works.

If you know the drivers, activities, and pressures, you can come up with measures to control them. When we ask what solutions we have, we look at the "10 tenets"—10 things you have to build in, including ecology, society, legislation, politics, administration, culture, morals, ethics, communication, and economics. We have the technology to stop contamination, but the conflict is often whether we have the economy to do it.

Stephen: I'm fascinated that conversations with environmental scientists start with ecology and quickly shift into values, politics, and philosophy. Estuaries are in constant flux. Can they teach us anything as a system about resilience, change, and our relationship with nature?

Mike Elliott: We used to think systems were delicate, but we now realize that when you remove pressures, they often undergo "passive recovery." However, sometimes that's not enough, and we have to do "active recovery"—building new wetlands to replace what we built cities on. For example, in Manila, they're building a new airport on wetlands, and to show they're environmentally responsible, they have to recreate wetlands elsewhere.

The challenge is whether we're doing it on a big enough scale. In the UK, the biggest scheme is about 800 hectares. In China, they're recreating hundreds of square kilometers. We have to create these because the biggest challenge to estuaries is climate change. With seas rising, changed flows, storminess, and higher temperatures, we have to give the sea somewhere to go. If we don't, we get "coastal squeeze," leading to flooding of urban areas. We protect large cities and industry, but what about farmland? If we don't protect certain land areas, we lose the emerald economy goods and benefits. The biggest challenge is this: how do we protect and maintain the natural system and, at the same time, deliver the human system? If we can get that balance, we've cracked it.

Stephen: I've learned so much from that. I'm going to finish off with two very quick questions we ask every guest at the Centre for Nature Recovery. First, what does "nature recovery" mean to you?

Mike Elliott: Nature recovery means we have put back the ecosystem services and societal goods and benefits we've lost. To get ecosystem services, you need the right structure and functioning. If we've put those things back that we've lost over hundreds of years, then nature is recovering, and we're protecting the biodiversity we've done a lot of damage to.

Stephen: Fantastic. And the last question: if I could give you a carbon-neutral flight to any ecosystem or area of the world, where is your special nature place that you would go?

Mike Elliott: I've been to too many places, and I dread to think what my carbon footprint is! Though fortunately, one of our sons worked for the Forestry Commission in Scotland and told me how many trees he'd planted, which offsets it a bit. I think I would like to go to places I haven't been to. I've been to the Arctic, but I would like to go to a place like Svalbard, because those areas are being affected by climate change much more than the rest of us. Ice is getting less, shipping channels are opening, and there'll be more ports developing. So it's places like that. I've still got a few places on my bucket list.

Stephen: Well, Professor Mike Elliott, thank you very much for educating us, informing us, and entertaining us. Thank you very much for your time.

Mike Elliott: That's great, my pleasure, Stephen. Thanks a lot.