Lagoons

These specialist habitats are essential to some of the world’s most unusual species.

For many the salty world of a saline lagoon is a hostile environment, but for the species that live in them, they are a perfect ecosystem that no other habitat can match.

These delicate habitats with their finally balanced levels of salinity are easily upset by environmental changes and their deterioration can have serious impacts on biodiversity.

Introduction to saline lagoons

What is a lagoon?

Saline or salty lagoons are areas of shallow coastal water separated from the sea, either wholly or partially, by sandbanks, shingle, rocks or man-made structures like sea defences. The water tends to be brackish with a salinity somewhere between freshwater and seawater. The level of salinity depends on local rainfall, fresh seawater from storms or temporary tidal flooding. Lagoons can be natural or manmade.

Where can you find lagoons?

Lagoons are a rare habitat but can be found all over the world. There are around 360 saline lagoons covering 5,200 ha in the UK.

Coastal lagoons are sheltered by sandbars or barrier islands and are created as a shallow basin where the shore gradually erodes, allowing sea water to seep in between the sandbars or barrier islands.

Atoll lagoons are similar to coastal lagoons but instead of being sheltered by sandbars or barrier islands they’re protected by coral reefs. They’re most commonly found in the tropical waters of the South Pacific and Indian Oceans.

What lives in a lagoon?

In the UK 36 species are particularly associated with lagoons, with 25 confined to them - four plants, 20 invertebrates and one bird – the avocet. These are some of the rarest plant and animal communities in the UK and some species are only found in one or two lagoons. A few species are also found in estuaries, but many ‘lagoon specialists’ are rarely found outside of lagoons.

The range of species found at each site will depend upon the salinity of the individual lagoon, making each one a unique ecosystem. Coastal lagoons support a range of salt-tolerant invertebrates. Some of these specialist lagoon invertebrates are protected by law under Schedule 5 of the Wildlife and Countryside Act.

Why we need lagoons

Saline lagoons are a priority habitat in the UK because they support so many specialist species. Centuries of coastal management and sea defence construction has led to the decline of natural lagoons and a reduction in quality of those remaining.

Coastal lagoons are highly productive ecosystems, which means they literally produce a lot of species in terms of volume and diversity. They contribute to the overall productivity of coastal waters by supporting a variety of habitats, including salt marshes, seagrasses, and mangroves. They also provide homes for many fish and shellfish species providing food and a livelihood for millions of people worldwide.

Lagoons in culture and history

Lagoons feature often in art and literature, their mysterious shapes taking form in works by Matisse and Paul Klee. In J M Barrie’s original Peter Pan story, the famous mermaids are only found in Neverland’s secret lagoon. They are portrayed as beautiful, haughty and mischievous, but at night they take on a darker role and sing songs to the moon at midnight.

More recently, the novel Where the Crawdads Sing celebrates the mysterious network of saline lagoon, swamp, marshland and coast on the North Carolina coast, bringing this overlooked landscape to life through the story of the ‘marsh girl’ who becomes an expert on it. It explores the little-known value of wetlands through fiction and the main character spends her time telling the stories of these places through her books so others can appreciate their worth.

Threats to lagoons

Pollution

Industrial and agricultural run-off, sewage outfalls, illegal dumping of rubbish – these can all have disastrous effects on lagoon ecosystems, which are often enclosed leading pollution to build up in lagoons with no outflow of water.

Development

Creates coastal squeeze and stops natural inland extension of saline lagoons, which may then become infilled. Drainage of surface water from nearby building developments can alter the salt levels of the lagoon.

Coastal defences

Interruption of natural coastal processes can stop new lagoons forming or cause the destruction of existing ones as natural shingle barriers degrade.

Sea level rise

Coastal lagoons can be lost to rising sea levels if the sea breaches the natural barriers that separate it from the lagoons.

What is WWT doing to protect lagoons?

If the habitats and species they support are to be maintained, an estimated 60 ha per decade need to be created to offset current losses. With much of the coastline now protected from erosion by sea defences new lagoons no longer naturally form so they need to be created.

WWT has a long history of maintaining wetlands and developing new ones. We also manage the habitat on our reserves to support a diverse a range of species. With this in mind we created the saline lagoon at WWT Washington in 2013. It was designed to improve WWT Washington’s biodiversity by strengthening its connection to the River Wear. Even when the tide is low, pools of water remain and this brackish habitat provides the perfect salinity levels for a unique range of wetland species including little egret, spoonbill, curlew, kingfisher and avocet to name a few – plus roe deer, Eurasian otter and an abundance of invertebrates and flora

Interruption of natural coastal processes can stop new lagoons forming or cause the destruction of existing ones as natural shingle barriers degrade.

We have started a freshwater lagoon restoration project at our sites in Arundel, and at Llanelli the water and salinity levels are carefully managed to provide a refuge when the estuary is inaccessible. We carried out some restoration work in 2021 and the invertebrate report showed encouraging signs. Birds feed on these creatures, and they also find shelter roosting along the edges of saline lagoons during high tide, until the estuary mud is exposed again.