What are salt marshes & why are they important?

We humans have a habit of thinking in terms of opposition: there’s rain or shine, hot or cold, and land or sea – to name but a few. Salt marshes, however, are something in between.

These unique habitats between earth and coast have, throughout history, been seen as somewhat liminal spaces. In literature and folklore, they’re often recorded as mysterious and mystical – even as places where the lines between natural and supernatural worlds are thin.

Now, don’t get us wrong, we love that salt marshes are the subject of poetry and intrigue. But perhaps we need to give them their due spotlight as real places of beauty, biodiversity, and ecological significance, too! 

So, let’s dive right into some of the most interesting facts about salt marshes, why they’re important, and what we can do to protect them. 

What are salt marshes? 

Salt marshes are intertidal wetland ecosystems found between uplands like forest and urban areas and lower estuaries or sheltered, low-energy coastlines. 

These wetlands are flooded at high tide and drained at low tide and, for this reason, are dominated by specialist salt-tolerant plants. The ebb and flow of water makes them marshy, with the soil generally composed of deep mud and peat – the latter of which is made of decomposing plant matter.  

The shoreline of a salt marsh typically consists of mud and sand flats, which you may hear referred to as tidal or mud flats. Sediment is washed into these flats by inflowing streams, rivers, or the ocean tide. 

Salt marshes occur worldwide but there is surprisingly little information out there about their rate of occurrence. Interestingly, one 2017 study mapped a total of 5,495,089 hectares of salt marsh across 43 countries and territories and marked this as a conservative estimate. 

But while 5 and a half million hectares sounds like a lot, that’s less than the land mass of Ireland – and they’re at threat, too, with estimates suggesting we’re losing 100 hectares per year. Durham University predicts that in the South East of England alone, our salt marshes could disappear entirely by 2040 because of rising sea levels, drainage, and development. Before we get into the threats they face, though, let’s explore just what makes these habitats so special.

Salt marsh ecosystems 

The characteristics of a salt marsh are truly unique. Because of their changing nature, they’re occupied (or visited by) many specialist species adapted to them. 

For lots of fish and shellfish species, salt marshes provide nurseries in which to breed and feed. Wetland invertebrates, too, thrive in the peat and are a great source of food for larger mammals like otters. As for birds, you’ll often find salt marshes positively teeming with wildfowl and waders that feed on insects, larvae, and small fish. 

To many, the resident and migratory birds that rely on salt marshes are among some of the most exciting species to be found there. In England, birds of prey like the short-eared owl, peregrine falcon, and marsh harrier can be seen using these habitats as hunting grounds, while nightingales, cuckoos, and ringed plovers visit to breed and nest.  

Examples of salt marshes 

Wherever there are low-energy intertidal zones, there are salt marshes – wherever they haven’t been drained and built over, that is! 

From the Punta Azufre Salt Marsh in Baja California and the Alaskan salt marshes further afield to the Stiffkey and Mawddach marshes closer to home in the UK, there are salt marshes across the world. 

Some examples in the Southeast of England, specifically, include: 

How are salt marshes formed? 

Salt marshes form where mud and silt built up in sheltered areas of coastline. This build-up of fine sediment becomes what’s known as a mudflat and, over time, it grows in size and elevation. As a result, flooding of the area becomes less intense – and plants begin to colonise it. 

Flooding continues to bring nutrients and sediment to the salt marsh and more plants move in, in a process known as ‘vegetation succession’. As plants decompose, they contribute to the peat base of the marsh which often extends several feet deep and absorbs salt from the tides. As a result, the peat is hypoxic (low in oxygen), and the growth of certain bacteria produces the sulphuric ‘rotten egg’ smell typical of salt marshes. 

Why are salt marshes important? 

Beyond being beautiful, often mysterious, and stark landscapes, salt marshes are globally significant habitats for many reasons. Here are just a few: 

  • They protect shorelines from erosion and storm flooding by buffering waves and trapping sediments. 
  • They reduce flooding in general by absorbing rainwater and slowing the spread. 
  • They protect water quality by filtering runoff from urban areas, acting as natural pollution filters and metabolising excess nutrients. 

  • They support biodiverse wildlife populations, including migratory birds and specialist plants. 

  • They are natural carbon capture zones, storing carbon both in the plants and sediment. A hectare of salt marsh can capture two tonnes of carbon every year and store it for millennia if undisturbed. 

The state of salt marshes & their conservation 

Throughout human history, salt marshes have been threatened. As far back as Roman times, marshes are recorded as having been ‘reclaimed’ from the ocean for use as agricultural land. In modern day, urban development is narrowing the gap between land and sea, pushing out salt marsh habitats. Further still, increasingly harsh storms and rising seas due to climate change are eroding salt marshes the world over.  

It’s no wonder, then, that we have lost an estimated 25-50% of the world’s historic salt marshes.  

But as more work is done to study their importance, more people are looking for effective conservation strategies. The good news is that salt marshes typically tend to need little management if we only let well alone.  

Where damage has been done, though, or where salt marshes could be ‘working harder’ with human intervention, restoration work can do wonders. The Essex Wildlife Trust, for example, established a salt marsh restoration project in collaboration with the Environment Agency at Abbotts Hall nature reserve, Blackwater estuary and Moverons Farm, Colne estuary. They have been strategically placing large coir rolls in low-energy channels within the salt marsh to encourage the build-up of sediment. Since the start of the project in 2018, the added structures have become integral parts of the marsh, stabilising it with the roots of new growth and sediment. 

Because they’re so invaluable as natural pollutant filters and carbon sinks, many researchers are calling not just for their restoration, but for their creation. The Wildfowl & Wetlands Trust, for one, is aiming to create a network of 100,000 hectares of wetland – including salt marsh – across the UK in order to help meet carbon targets. Their existing reserve, Steart Marshes, currently stores an impressive 10,000 tonnes of carbon every year. As well as that, of course, it makes space for nature, reduces flood risk, and benefits the wellbeing of local communities.  

We certainly hope there’ll be even more emphasis put on just how crucial salt marshes are – and that this is just the start of a conservation movement.  

Visit Sandwich & Pegwell Bay 

To experience the beauty of salt marsh for yourself, be sure to take a trip to Sandwich & Pegwell Bay! This precious salt marsh is well worth a visit at any time of year – be it a bleak, grey day when the landscape stretches out dramatically before you, or a warm morning spent watching the sun rise over the sea as the world and its wildlife wake up. 

It’s one of the largest land haul-outs in Kent for seals, so you might be lucky enough to see some common or visiting grey seals. Take your binoculars, too, so you can watch the wading birds feed on the mudflats. 

It seems fitting, in a blog about salt marshes and their importance, to talk about the fact that the future of Sandwich & Pegwell Bay is at risk at the time of writing. National Grid’s Sea Link plans involve a new cable making landfall at this internationally important National Nature Reserve with Site of Special Scientific Interest (SSSI), Special Protection Area (SPA), and Ramsar designations. The converter and sub stations will be built within Minster Marshes, resulting in the loss of 13ha of valuable wildlife-rich habitat.   

If our words have rung true for you, we invite you to join us in calling on them to Rethink Sea Link – read more here. 

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