Salt Marshes: From Mudflats to Ecological Paradise

At the border between land and sea, is an ecological wonder of birds, shellfishes, and salty wild plants: salt marshes, a coastal biodiversity hotspot and a natural climate-mitigation site. In this article, we explore what salt marshes are and what makes them so special.

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Salt marshes are intertidal wetlands – coastal ecosystems found in areas between land and open saltwater. Given their proximity to the sea, these ecosystems are regularly flooded by tides. However, due to their elevation, they remain safe from intense flooding, and undergo small and frequent tidal events.

Their elevation originates from the accumulation of sediments coming in from the sea, entering mudflats over time. The accumulation of sediments and the presence of tides creates environmental conditions for salt-tolerant species to thrive. With time, these species colonise the mudflats and, in the presence of vegetation, slowly turn them into salt marsh ecosystems.

Common plant species found in salt marshes are succulent samphire (Salicornia europaea), cord-grasses (Spartina anglica), sea purslane (Spartina anglica), sea aster (Aster tripolium) and sea lavender (Limonium vulgare). Interestingly, these species will be found in different parts of the salt marshes, depending on soil moisture and other ecological conditions. The distribution of these species along an environmental gradient is known as species zonation. Such zonation of species and ecological processes make the salt marsh a dynamic and patchy habitat.

Salt marshes are estimated to cover almost 5.5 million hectares of the world. They can be found in many coasts around the world, such as Australia, throughout the US, in Europe or China, and even Madagascar. Unfortunately, these important habitats are in decline globally, declining at an annual rate of about 100 hectares.

Nearly 30% of all salt marshes are located within the US, with a famous hotspot observed along the coasts of North Carolina. 

In the UK, 85% of salt marsh in England has been lost since 1860, with only about 45,000 hectares remaining today that are increasingly prioritised as important sites of ecosystem conservation. A 2023 study suggested that saltmarsh habitats in the UK could disappear completely by 2040 due to increasing threats such as sea level rise and coastal infrastructure development. 

Ecological Importance

But why should we care about salt marshes, and what makes them so special?

Salt marshes are very interesting ecosystems that provide multiple ecological processes and services.

A shelter for some species… 

Salt marshes are home to a biodiverse array of fauna and flora. In addition to the aforementioned salt marsh plants, fish, shellfish, and wetlands invertebrates can be found living in these ecosystems. Birds like nightingales, cuckoos, and ringed plovers also use salt marshes as a location for breeding and nesting. 

A food supply for others…

While some species use salt marshes as nesting and breeding sites, other species feed on them. These include otters feeding on invertebrates, or birds such as the short-eared owl, the peregrine falcon, and the marsh harrier, with the latter hunting in salt marshes.

A natural barrier to flooding… 

Salt marshes are natural barriers against the increasingly strong water events occurring on coastal landscapes. Their presence along the coast reduces flooding through the absorption and slow release of rainwater. Salt marshes have been found to reduce property damage by up to 20% during storms. 

Not only do they protect against flooding but they also act as great water filters, by filtering runoff from urban areas, allowing pollutants and excess nutrients to be absorbed and processed.

And a blue carbon pool …

Blue carbon refers to the natural carbon stored within marine ecosystems. With discourses around the climate crisis increasingly focusing on carbon capture, more research is focusing on existing and natural ways to store carbon and prevent its release in the atmosphere. This is why naturally occurring carbon storage ecosystems such as salt marshes are of great importance. Research shows that salt marshes and coastal wetlands have a 10 times greater capacity at sequestering and storing carbon than mature tropical forest. If undisturbed, an hectare of salt marsh, through its vegetation and sediments, can sequester up to two tonnes of carbon annually, and keep it stored and intact for millennia.

Cultural Values

Salt marshes’ incredible characteristics go beyond ecological values. You may have heard of them, from folklore legends to settings to famous novels. But the place of saltmarsh in our anthropogenic cultures goes beyond fictional stories.

Various plants from salt marsh ecosystems are edible and have been used for thousands of years in our kitchen. Sea purslane, for instance, can be used as a salty and delicate herb to serve with fish dishes. 

Today, one of the main ways that salt marsh plays a role in our diet is through their use as salt production. A famous salt production location, l’Ile de Ré on France’s west coast, has been targeted for salt extraction activities since the Middle Age. But despite its popularity and importance in local culture, such salt extraction activities from natural ecosystems can severely damage ecosystems, contributing to their decline. This is an example of the challenges faced today when trying to protect our natural environment. Cultural and economic objectives can sometimes go against ecological health, highlighting the importance of understanding the full use and significance of an ecosystem within its embodied socio-ecological context. 

The San Francisco bay history of salt production is a good example of such human-led degradation. The beautiful wetlands of the bay used to be a nest of species and biodiversity, until industrial-scale salt extraction began in the mid-1800s, altering some 80-85% of the original coastal ecosystem. In 2002, following rising concern and increasing awareness of the health of the wetlands, the South Bay Salt Restoration Project began, targeting 6,000 hectares of salt marshes. To this day, the project has successfully reconnected degraded marshlands and contributed to recovering bird populations such as snowy plovers or american avocets. 

Threats and Future

Salt extraction is not the most prominent threat facing salt marsh habitats. Sea level rise, increasing intensity and frequency of storm events, and drainage of wetlands for agricultural use or urban extensions are among the main challenges these precious ecosystems face today.

Combined together, urban expansion and sea level creates a phenomenon known as “coastal squeeze,” where coastal habitats such as salt marshes are disappearing, squeezed up between land development and rising seas. 

There is great concern about the remaining salt marshes of the world and an urgent need to raise awareness and action towards their protection.

Thankfully, in recent years, increasing examples of salt marsh restoration have taken place around the world, giving hope to the future of salt marshes. An example of such restoration can be found at the Blackwater Estuary in Essex, England. The project focused on destroying man-made sea defences (concrete wall) and removing farming activities bordering the coast. This way, enough land is left free for salt marsh habitats to form on the previously managed land and thrive.