Coasts

Coastal ecosystems

Coastal environments provide niches for vegetation and animal species to exist in that are not found elsewhere. Firstly, there is salt marsh. Stagnant or slow-moving water found behind a spit, or in a lagoon, can provide the perfect conditions for deposition of sediment to form mud-flats. Once enough material has been laid down for land to become exposed at low-tide, specialist plants can colonise the site. Known as pioneer species, they are specially adapted to the extreme conditions found in the inter-tidal zone. plants such as eel grass and salicornia can survive regular submersion by salt water, whilst are also tolerant of the drying effect of strong winds at low tide.

Over time, these pioneers help to trap sediments, allowing the height of the mudflats to increase. This is the start of a process of plant succession where less tolerant but more complex species can now invade as conditions ameliorate (improve) further. Moving inland, one can normally observe a series of changes as older, higher land is encountered where more time for change has passed. The sequence of changes is called a halosere and may often end, at some distance inland, with a mature deciduous forest - the climatic climax community of the UK.

Similar changes can be observed in coastal sand-dunes, known as a psammosere. Here, plants such as marram grass that can survive life in the fore-dunes are tolerant of extreme aridity and have long tap roots that can reach the water table. Further inland, the addition of humus (organic matter) from previous generations of marram has allowed species such as gorse or small trees to colonise wherever a stable, water-retentive soil has developed. Sand dunes also provide one additional niche for rare species - in between the dunes lie damp hollows known as dune slacks that can support hydrophytes (water-loving species), including insects and animals. For instance, the natterjack toad lives in the dune slacks at Formby and Ainsdale in Lancashire.

What impact will climate change have on salt marsh and sand dune ecosystems in the UK?

Halosere environments are at risk of submersion, especially along the south coast (e.g. the Solent) where geological processes are causing the land to sink at the same time as sea-levels are starting to rise. If salt marsh vegetation cannot trap new sediments at a rate that keeps pace with the net rise in sea level, then it may be completely overwhelmed and be lost to the sea.

However, a new strategy of managed retreat is being adopted by some land-owners. This involves farmland further inland being abandoned to the sea in order to provide a new 'buffer' against the rising tides. This means that halosere communities will not be lost in many places - they will instead migrate further inland as previously dry agricultural land changes into mud flats. For instance, along National Trust land in Porlock (in Somerset) the sea has been allowed to breach the shingle ridge. Now that it floods behind the shingle, a new saltmarsh is rapidly developing.

Psammosere environments may become more at risk of extreme storms which can cause blow-outs and destroy dunes. Recently, the sand dunes at Formby and Ainsdale have suffered as the result of a series of severe winter weather events. Severe storms can take twelve to fifteen metres from the front of the dunes. In January 2007, a major storm blew sand 25 metres inland over 48 hours, covering over tourist facilities such as the car park. Formby may have to contend with such extreme conditions on a more regular basis in the future. Over the next 100 years, the dunes may recede by more than 400 metres, a change which the National Trust says it will allow to occur naturally.

Student Practice Question:

Examine the factors influencing the development of a named coastal ecosystem

One criticism of the model of plant succession leading to climatic climax is that it assumes a stable geological and climatic environment for autogenic (plant-driven) changes to take place within. A good answer to this essay will describe and explain the general changes that have thus far been observed for either a psammosere of halosere. However, it may then question whether these processes will continue to operate effectively if sea-level change is also occurring: for instance, can the relative level of mud flats continue to be raised through deposition, if the height of the water is also starting to rise?