Estuary Habitats: Wildlife, Ecosystems and Why They Matter

Alongside our seas, oceans, and rivers, estuaries are often overlooked. But did you know that these are among the most precious marine habitats on Earth? 

Estuaries support a surprising diversity of life, from mammals and birds to fish and invertebrates. They’re home to invaluable flora like mangroves, seagrasses, and kelp forests, too, making them a key player in the fight against climate change.

Why not join us for a closer look at estuary habitats, as we share some of the unique animals and plants that call these waters home.

What is an estuary?

An estuary is the mouth of a river at the point where it meets the sea. The combination of freshwater and salt water creates a unique habitat for wildlife, while the influence of the tide means that estuaries provide a key role in protecting communities from coastal erosion and climate change. 

Three key characteristics of estuaries you need to know

Estuaries are among the most important wildlife habitats on Earth, while also playing a vital role in human geography. But what are the physical characteristics of estuaries that make them such an invaluable natural feature of our landscape?

• Brackish water: Estuaries are among the only places where you find brackish water, which comes as a result of the mixing of salt and freshwater. Brackish water has a higher salinity than freshwater but isn’t as salty as the sea, so many species have evolved to thrive in it.
• Nutrient-rich: The combination of brackish water and ocean currents means that estuaries are rich in nutrients. These nutrients help to support a wide variety of species, while also providing an optimal breeding ground for marine animals like salmon, eels, and sea trout.
• A natural tidal defence: Estuaries support a diverse mix of flora, including seagrass beds, mangroves and marshland. Not only do these provide a natural tidal barrier, protecting coastal communities, but they’re considered one of the world’s most effective carbon capture and storage providers.

The main types of estuary habitats

Estuaries support an astonishing array of life, so you’ll encounter different types of habitats around the world. Here in the UK, kelp forests, seagrass meadows, saltmarshes and sand flats are common, while we’ve also seen native oyster beds making a return in some regions.

Further afield, estuaries become more complex and exotic, supporting flora like mangrove forests and coral reefs, along with other aquatic marine vegetation. What links all these habitats is that they thrive in brackish water, which you’ll find at the mouth of rivers where they meet the sea.

Why are estuaries important?

It’s impossible to overstate the role estuaries play in the natural world. Not only do they support irreplaceable marine life, but they help to protect vulnerable coastal communities across the world, too. 

As alluded to earlier, many species rely on estuaries as their breeding grounds. That’s because they offer calmer waters, fewer predators, and optimal nutrition. Some estuaries are even used as “migration highways” for species like salmon, bridging the gap between salt and freshwater.

Humans benefit from estuaries as much as animals. Billions of people across the globe live on or close to estuaries, including in major cities like London, New York and Tokyo. That’s only possible thanks to the natural barrier against floods and storms these features offer.

Let’s take a closer look at why estuaries are more important than you might think.

Habitat security

Estuaries provide irreplaceable breeding, feeding, and migration grounds for a variety of marine life. Even ocean-faring species rely on estuaries for their sheltered waters and reduced predation rates, often returning to the same estuary time and again over their life cycle.

Estuaries are a natural sanctuary for many species of fish. They offer abundant food and nutrients, a calm area to serve as a nursery, and dense marine vegetation that offers protection from predators.

Of course, estuaries are varied, so they accommodate a huge number of species. Seals, salmon, seabirds – all visit brackish waters to feed, breed, and raise their young.

Natural water filtration

Estuaries act as natural water filters, removing pollutants and bringing vital nutrients to coastal areas. They also process and remove harmful sediment before it reaches the sea, helping to protect flora and fauna along the adjacent seaboard.

Protection against climate change

Did you know that estuaries are among the best natural carbon storage centres on Earth? They support flora like seagrasses, reeds and mangroves, which are among the most effective carbon capture vessels in our seas and oceans.

Not only that, but estuaries store a significant volume of water, helping to maintain predictable sea levels. They temporarily hold excess rain and tidal waters, too, reducing the impact on surrounding drainage systems.

Flood and storm protection

From tropical mangroves to mudflats and wetlands found right here in the UK – the natural features of estuaries offer invaluable protection against storms and floods. 

Think about it: estuaries support flora like mangrove forests, coral reefs, and reed beds, all of which are known to quell tidal surges. Mudflats, wetlands, and marshes absorb excess water, too, reducing the likelihood of inland flooding.

What wildlife live in estuaries?

By now you should have a pretty good idea of just how important estuaries are within wider marine habitats. Now it’s time to turn our attention to the species that dwell in these unique, brackish water bodies.

Fish

It’s estimated that over 2,000 species of fish live in or visit estuaries globally – far too many to list here! There are some big (quite literally) surprises in the mix, too, from the formidable bull shark and the beluga sturgeon, to the giant freshwater stingray, which can grow up to five metres in diameter.

Here in the UK, estuary-dwelling fish are a little on the smaller side. Species known to live in or visit estuaries around our coast include the European bass, the grey mullet, the sand goby, and the thornback ray.

Birds

With their shallow waters, natural vegetation, and all-you-can-eat buffet of small fish species, estuaries provide a home for all kinds of native and migratory birds. 

Globally, estuaries are home to some of the world’s most iconic bird species, from the American flamingo and the great white pelican to the African fish eagle and the roseate spoonbill. There’s no shortage of fascinating winged visitors to UK estuaries, either, including kingfishers, herons, oystercatchers, and avocets.

Invertebrates

Mud, sand, sediment, and shallow water – estuaries feature all the elements that invertebrates love. Indeed, they support more invertebrate species than any type of fauna, from crabs, prawns, and barnacles, to jellyfish, anemones, whelks, and sponges.

In UK estuaries, you’re likely to find invertebrates ranging from the common cockle to Baltic tellin. Worms also feature heavily in estuarine areas around our coasts, including ragworms, lugworms, and sand mason worms.

Mammals

Where there are fish, there are mammals, so it shouldn’t surprise you to learn that estuaries are home to species like Eurasian otters, bottlenose dolphins, and beautiful harbour seals. Indeed, brackish water supports some truly curious mammal species – from the West Indian manatee to the dugong, nicknamed the cow of the seas.

UK estuaries see their share of mammal-class visitors too, particularly seals, otters, and dolphins. One of the largest mammals often seen in estuaries is the minke whale, while orcas may occasionally swim into deep river estuaries too.

Estuary examples and fun facts

You’re close to knowing all there is to know about estuaries in the UK and beyond. But there’s still more to learn, so here are some fun facts to inspire you to keep exploring…

The Amazon’s Río de la Plata estuary is the largest on Earth

It carries 15% of all the world’s freshwater to the sea, discharging it at a rate of 90,000–100,000 m³ per second. And it’s been reported that the Amazon’s Río de la Plata estuary transports up to 1.1–1.3 billion tonnes of sediment per year. Incredible, right?

The Severn Estuary is among the world’s largest tidal ranges

That’s right – the Severn Estuary, found in southwest England, is the site of one of the world’s largest tidal ranges. Here, the tide can reach around 12-14 metres, hence why it’s become famous for its saltmarsh and mudflats.

The Humber estuary is revered for its wildlife internationally

Over 35,000 hectares of the Humber estuary in England are part of a protected wildlife reserve. Indeed, it’s considered one of the most important ecological wetland areas on Earth, and is protected by the Ramsar Convention.

The Upper Solway Flats is the largest estuary in the UK by area

It covers an impressive 43,637 hectares. That’s roughly the size of New Orleans in Louisiana, USA, as well as the Caribbean island of Barbados. Good to know, right?

UK estuaries change dramatically within a couple of hours

That’s as a result of the tide. For instance, in the Severn Estuary, the area transforms from sweeping sand flats to a shallow seawater habitat within a few hours. This cycle makes it a vital feeding ground for wading birds and invaluable marine microorganisms.

Connect with the wildlife of estuaries at Deep Sea World

Here at Deep Sea World, we’re uniquely connected to estuaries in more ways than one. Our aquarium lies on the Firth of Forth, one of Scotland’s most dramatic estuaries. Indeed, it sits in the shadow of the iconic Forth Bridge, which was designated a UNESCO World Heritage Site in 2015.

Our estuarine location is reflected in our exhibits, too. From Mangrove to Brackish, we showcase the unique flora and fauna of the world’s estuaries, including species like the figure-eight pufferfish and the silver moony. 

So, if you’d like to experience the surprising beauty and diversity of estuaries for yourself, book your tickets to DSW today. We can’t wait to welcome you.