What are saltmarshes?

In this blog post, we want to explain why saltmarshes are an integral part of the coastal ecosystem, what important functions they fulfil, and why it is important to protect them.

Saltmarshes are coastal wetlands, highly adapted wetlands, that are able to deal with the dynamic nature of the tides and the salty conditions of sea water. Saltmarshes can be found around the world, in particular around estuaries, often looking like wet grassland, interspersed with creek networks and tidal pools. Saltmarshes usually follow a profile (Figure 1 below) where the slope slowly increases landwards, building up sediment that is deposited with the tides on the higher elevations, gradually sloping towards a transition zone where there is minimal tidal influence.

However, many saltmarshes and estuaries have undergone major transformations in the past. Many saltmarshes have been drained to allow intensive grazing, others have been lost due to erosion and coastal squeeze – a process where estuarine habitats are lost due to increased pressures from the water because hard sea defences have been built in the vicinity.

Figure 1. Profile diagram of a typical northwest European saltmarsh, illustrating the relative positions of some of the most abundant species (modified from Burd 1992), as published in Hughes, R.. (2004).
Figure 2: Saltmarsh edge

Saltmarshes in the Mersey estuary

The Mersey estuary is a good example of a system where saltmarshes were once broad and wide, but with time and development have become smaller and constricted. In the Upper Mersey, the area MGET are regularly working in, the saltmarshes are lacking the typical slope of an expansive marsh, mainly being ‘Upper Marsh’, with only small areas where we can see a gentle slope and typical development of a pioneer zone.

We should, however, not forget that the saltmarsh that we refer to as Widnes Warth and Astmoor Marsh are 17 miles (27km) inland and that we are incredibly fortunate to have this habitat on our doorstep.

Why are saltmarshes important? Why is it so important to protect them?

Saltmarshes have an incredibly important role in the ecosystem. They perform functions that are globally important for climate regulation and carbon sink, but are also important locally, as they act as a flood plain and biodiversity hotspot.

Water storage

The primary ecosystem functions for saltmarshes have to do with water. A saltmarsh is essentially a sponge area. Just like flood plains in a freshwater context, saltmarshes buffer the impact of incoming water from the tides, which make the water retention potential of the saltmarshes an important function. With rainfall events becoming more extreme, the buffering capacity of the saltmarshes becomes a very important feature.

Figure 3: creeks storing water
Figure 4: Saltmarsh plants

Carbon storage

Although we do not have exact values of carbon storage potential for our local saltmarshes, a 3-year study suggests that the carbon content of our local saltmarshes is comparable with UK datasets, which estimate that saltmarshes in the UK store on average 115 Mg (Megagram = approx. 1 metric tonne) of Carbon per hectare (Lima, 2025). This is an important feature of saltmarshes, as they are similar to other wetlands, where plants are using CO2 for their growth, and any dead plant material is buried in waterlogged conditions, breaking down incredibly slowly, preventing the carbon being released into the atmosphere again.

Important habitat

Our local saltmarshes are also important for many species of breeding and overwintering birds, small mammals like rodents, and larger mammal species like otters and badgers, many of which are nationally declining or are already recognised as vulnerable. Saltmarshes are not (and shouldn’t be) accessible habitats, which makes it difficult to know what’s present and needs protecting. This is why we put a lot of effort into recording what is out there and supporting the existing functions of the marsh to be strong and resilient.

Figure 5: Redshank, a ground nesting bird

We are sometimes asked if nature would be best left to fend for itself, and why we need to intervene. This is a complex topic and we’ll discuss this in another blog post, but the short version for this article is: if we want to support and maintain the current functions of the habitat and make the habitats stronger for future change, we need to help out, because the human-induced pressure on the environment is too high for habitats to keep up by natural adaptation.

What MGET do on the saltmarshes to support its function

One of the first things to do when wanting to support any habitat is to understand the habitat: how it works, it’s strengths, its problems, its species, and its place in the whole ecosystem around it. This is not an easy task and can take years to accomplish. Even with a thorough data set, it is often impossible to fully understand all interactions in an ecosystem. And then we also face the challenge of deciding which element, i.e. which habitat type, which species, and which function should be supported. Sometimes this means having to accept that the support of one part could mean a decrease of presence or function of another part of the saltmarsh.

At MGET, we do our very best to collect thorough data to support our interactions. For example, as part of an Environment Agency Local Water Environment Grant funded project, we looked at the creek networks on Astmoor and Widnes Warth saltmarshes. We studied their shape and flow dynamics and identified several creeks which would benefit from an intervention. These interventions would either slow the flow and enable the tidal water to stay on the marsh for a longer period of time, or enable sediment to settle in the creeks, slowing the erosion within the creeks, and simultaneously providing habitat for invertebrates, crustaceans and small fish.

Figure 6: Brushwood bundle to slow trap sediment in creek.
Figure 7: Coir rolls across creek, slowing the flow.

There is no definite answer to how saltmarsh should be managed for nature, but one thing is certain, nature will lead the way and if we observe carefully and are willing to work with it, we can improve outcomes for humans and nature.

Figure 8: View over the estuary from Wigg Island

References

Hughes, R.. (2004). Climate change and loss of saltmarshes: Consequences for birds. Ibis. 146. 21 – 28. 10.1111/j.1474-919X.2004.00324.x.

do Amaral Camara Lima M (2025) Living Links: Coastal Vegetated Ecosystems in UK Estuaries as Blue Carbon Sinks and Food Web Foundations. Earth Sciences. IntechOpen. Available at: http://dx.doi.org/10.5772/intechopen.1012186.