Habitats

Habitats

The Severn Estuary supports some of the most important and protected habitats in the UK, with its vast tidal range playing a major role in creating the unusual physical conditions of the Estuary. Its unique set of physical characteristics create a highly dynamic environment and support a wide range of communities. The Estuary hosts three different categories of habitat: subtidal, intertidal, and supratidal. Subtidal habitat is permanently submerged in water, whereas supratidal habitat covers dry land. Intertidal habitat is covered with water when the tide is high and is exposed to the air when tide is low.

The intertidal zone of mudflats, sandbanks, rocky platforms, and saltmarshes is one of the largest in the UK. This diversity of habitats allows the Estuary to support internationally important numbers of waterfowl and large numbers of aquatic invertebrate populations. The Estuary also provides a valuable corridor for migratory fish and acts as a key nursery area for many species (see section on fish).

Saltmarsh

The Severn Estuary contains the largest area of saltmarsh habitat in the south and southwest area of the UK, covering about 1,400 ha, representing 4% of the total area of saltmarsh in the UK1. Saltmarshes are constructed from halophytes (plants adapted to live in saltwater) which thrive in the salty estuarine environment of the estuary. Plants such as glasswort, common reed and sea barley colonise these marshlands as well as nationally important rare species such as the bulbous foxtail2. Numerous Species of European Conservation Concern (SPEC species), such as the redshank, and shelduck also make a home in the Estuary’s saltmarshes. These birds make use of the saltmarsh plants that shelter them from the tide and protect their young. Extensive networks of freshwater and brackish water (a mixture of saltwater and freshwater) drainage ditches sustain water voles and aquatic invertebrates.

Saltmarsh environments fringe the coastline and can be found at locations such as Andrew’s Pant, near East Aberthaw and Clevedon Pill. Sites like this are of international importance, due to the delicate balance of environmental conditions which sustain the saltmarsh. As such, they are designated as an Annex 1 Habitat3 and included in the Severn Estuary Special Area of Conservation (SAC)4.

Intertidal mudflats

Mudflats and their accompanying sandflats are usually found within the middle reaches of the Estuary and comprise a large proportion of intertidal habitat. These environments are submerged at high tide and exposed at low tide. They are divided into three main types: clean sands, muddy sands and mudflats5. Within the most open areas of the Severn Estuary, such as the Bristol Channel, are the clean sands, where ocean waves and tidal currents prevent the deposition of the finest silt. Due to the more choppy and high-energy nature of this area, crustaceans found in this area tend to be sturdy. Bivalve molluscs and tiny sandhoppers can adjust to the dynamic nature of this environment. Muddy sands occur in more sheltered areas and have a wide range of species such as lugworms, mussels, and eelgrass. Mudflats form in the most sheltered areas of the coast, where there are large quantities of silt deposited by rivers such as the Severn. These habitats are more stable and calm. Worms and mud-snails inhabit the fine muds and provide food sources for bird waders such as the common shelduck, knot and dunlin. Despite the vast expanse of the Severn mudflats, the overall species diversity (i.e. number of different species) is low, and smaller numbers of more ‘specialist’ species inhabit the mudflats.

These intertidal mudflats are of major conservation importance for species in the UK. They support the fish assemblage sub-feature of the Severn Estuary Special Area of Conservation (SAC) and Ramsar site, as well as the bird wintering and passage features of Special Protection Area (SPA) and Ramsar sites (see section on Nature Conservation Designations).

Rocky intertidal habitat

Rocky intertidal areas are just what it says on the tin. They are made up of hard materials like rock! There are extensive rocky shores along the estuary, made up of rock, boulder, mussel/cobble scars, rocky pools, and shingle. They represent an extreme environment where harsh conditions prevail due to exposure to tides, waves, and wind. However, the variety of ground materials creates a large range of habitats for a wide range of species to inhabit. The rocky shores of the Severn support many plants and animals, including brown and green algae, barnacles, limpets, and winkles. They also provide an essential food source and resting place for a wide range of wintering and migratory waterfowl, such as dunlin[EM1]  and redshank. Bird species such as the knot, oystercatcher, and curlew (see section on Birds) create roosting sites in the cracks and crevices of rock.

Rocky intertidal areas are a key ‘supporting habitat’ for the conservation of many important species. Due to the internationally important species they support, especially migratory birds, the habitat is of European significance.  

Seagrass beds

Seagrass habitats are made up of eel grasses which occur within the more sheltered areas of the Estuary such as the Welsh side of the Second Severn Crossing. These underwater plants are adapted to the saline (salty) conditions of the seawater and grow to form large ‘meadows’. The two recognised species in Wales, the dwarf eelgrass, and the eelgrass (also known as seawrack), are restricted to intertidal and subtidal habitats respectively6. Unique to the Severn Estuary, seagrasses inhabit a wide range of ground material, such as cobbles, sands, muds, and large boulders. Elsewhere, seagrasses are more particular and usually only grow on mudflats.

The grasses provide a productive habitat for a wide range of species, including algae, invertebrates, and fish. The seagrass creates a form of protection for smaller invertebrates, such as molluscs and urchins, by stabilising the sediment beds in which the invertebrates burrow6. They also provide a habitat for overwintering birds, such as geese and duck, and a valuable food source to these important migratory species.

Biogenic Reefs

Reefs in the Severn Estuary are called ‘biogenic reefs’ because they are constructed by a worm. Much like the honeybee constructs its hive, dwelling polychaete worms known as the honeycomb worm construct tubes to live in! Using sediment and sand particles, the honeycomb worm creates a honeycomb-like collection of tunnels. These clever worms live in both intertidal and subtidal areas. These reefs distinguish the region from other estuaries in the southwest of the UK, where such reefs are largely absent.

Intertidal reefs can be found at locations such as Redcliff Bay near Portishead, south of Severn Beach near Avonmouth, and at Goldcliff near Newport. Subtidal reefs occur in dense, large colonies throughout the mouth of the Estuary7. The reef structures provide habitats for many other species. Within secluded tunnels and crevices, crabs, periwinkles, worms, barnacles, and anemones inhabit reef structures. Subtidal reefs also support colonies of brittle stars and shrimp. Due to their importance in providing a habitat, and the unique conditions of the estuary, these reefs are protected as part of the Severn Estuary SAC ‘Reefs’ feature.

Subtidal Sand banks

Subtidal sandbanks exist in shallow depths of less than 20 meters, and as their name suggests, are permanently submerged in water. These sandbanks are ‘mobile’ and can move due to the powerful tidal range within the Estuary, which restricts the level of biodiversity within this environment. These subtidal sandbanks can be divided into four subtypes: gravelly and clean sands, muddy sands, eelgrass beds and maerl beds (composed of free-living red algae)8.

The sandbanks of the Severn Estuary are not only distinct from other estuaries, but they are some of the best in the UK. In fact, invertebrate communities such as crustaceans, echinoderms (such as starfish) and lugworms all inhabit this dynamic environment. Larvae from different species use the moving sand banks to their advantage and can spread to other areas in the estuary.

Sandbanks are an Annex I qualifying feature of the Severn Estuary SAC under the Habitats Directive and are also associated with other Annex 1 habitats. Subtidal invertebrate communities within these sandbanks help to provide a source of food and spawning grounds for fish such as sand-eels (which are in fact, a type of fish and not an eel) under the SAC and Ramsar site designations. Feeding waterbirds such as razorbills and guillemots8 are also able to access the shallow sand banks to hunt for food.

Climate change

Rising sea levels pose a variety of challenges. Coastal squeeze occurs when both rising sea level and ‘human activity’ (buildings, flood defences, roads etc) squash the coastal habitat between the sea and the built environment. This can mean that, over time, coastal habitats have a smaller and smaller area to inhabit. Habitats which are affected by coastal squeeze include beaches, seagrass and reed beds, saltmarshes, mudflats, and sand dunes10. Climate change also affects reef habitats, especially in the long term. The honeycomb worm builds its reefs its northern most range in the UK. The worm tends to prefer warmer temperatures, with the UK’s cool waters being as far north as the worm ventures. With increasing sea surface temperatures, these reef building species could benefit from a slight increase in water temperature and extend their range further north. For more information about the effects of climate change within the Severn estuary, see ‘Weather and Climate Change’ section.


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References
  1. Dargie, T. 2000. Description of the Severn Estuary Survey Sectors Identified in the 1998 NVC Survey. CCW Contract Science Report No. 399.
  2. Plantlife: Severn Estuary Shore IPA webpage https://www.plantlife.org.uk/uk/nature-reserves-important-plant-areas/important-plant-areas/severn-estuary-shore (2021)
  3. Joint Nature Conservation Committee: Severn Estuary/ Môr Hafren webpage – https://sac.jncc.gov.uk/site/UK0013030
  4. Severn Estuary / Môr Hafren Special Area of Conservation: Indicative site level feature condition assessments. Evidence Report No: 235 (Natural Resources Wales, 2018) https://cdn.naturalresources.wales/media/684391/severn-sac-ica-2018.pdf
  5. Joint Nature Conservation Committee: Mudflats and sandflats not covered by seawater at low tide webpage – https://sac.jncc.gov.uk/habitat/H1140/
  6. Benthic habitat assessment guidance for marine developments and activities: a guide to characterising and monitoring seagrass beds. GN030f (Natural Resources Wales, 2019) https://cdn.cyfoethnaturiol.cymru/media/693114/gn030f-seagrass-final-24jun2019-accessible.pdf
  7. Severn Estuary Regulation 33 Advice, Appendix 6 Map showing the extent of the “reef” feature of the Severn estuary SAC. (Natural England, 2009)
  8. Joint Nature Conservation Committee: Sandbanks which are slightly covered by sea water all the time webpage – https://sac.jncc.gov.uk/habitat/H1110/
  9. Severn Estuary Coastal Habitat Management Plan (Environment Agency and APBmer, 2006)
  10. What is coastal squeeze: report – https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/955318/FRS17187_What_is_coastal_squeeze_-_report.pdf (Environment Agency, 2021)

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