Habitats

1. Estuaries.

2. Large shallow inlets and bays.

3. Atlantic salt meadows

4. Salicornia and other annuals colonising mud and sand.

5. Mudflats and sandflats not covered by seawater at low tide.

6. Sandbanks which are slightly covered by sea water all the time.

Estuaries

Estuaries are partly enclosed bodies of water at the mouths of rivers which are subject to the tide.  They are thus open to saline water from the sea and fresh water from the rivers.  They are complex environments with a range of habitats grading from riverine to marine.  Some of these habitats, such as intertidal mudflats and sandflats, Atlantic salt meadows and Salicornia and other annuals colonising mud and sand have been included in Annexe I of the Habitats Directive as habitat types in their own right.

The Carmarthen Bay and Estuaries cSAC has four estuaries within it's boundaries: the Loughor, Taf, Towy and Gwendraeth.

There are four main types of estuary:

1. Coastal Plain Estuaries.  These are typically wide and shallow estuaries formed by the flooding of pre-existing valleys at the end of the last ice-age. 

2. Bar-Built Estuaries. These are quite widespread around the UK, they have a sediment bar across the mouth of a partially drowned river valley.

3. Complex Estuaries.  Quite rare in the UK, these are formed by various influences such as sea level change, erosion and geological constraints from hard rock outcrops.

4. Ria Estuaries.  Drowned river valleys with estuarine features restricted to the upper reaches.

The Loughor, Taf and Towy are all coastal plain estuaries, while the Gwendraeth is a bar built estuary.

The intertidal (covered/uncovered by the tide) and the subtidal (below the intertidal and thus permanently covered by the sea) sediments of estuaries support important biological communities.  These communities vary according to geographic location, the type of sediment, tidal currents and salinity gradients within the estuary.

Large Shallow Inlets and Bays

Large shallow inlets and bays, as the name suggests, are large indentations of the coast, often more sheltered from wave action than the open coast.  They are shallow (usually less than 30m deep).  There are 3 main types in the UK:

1. Embayment.  A marine inlet where the coastline usually follows a concave curve between rocky headlands.

2. Fjard.  A series of shallow basins connected to the sea by shallow, sometimes intertidal, sills.  They do not have the U shaped cross section or steep cliffs of fjiords.

3. Ria.  A drowned river valley in an area of high relief (an area with steep slopes).  These are known as Voes in Scotland.

Carmarthen Bay is an embayment (of which there are few  around the UK coast).

Large shallow inlets and bays, like estuaries, are complex systems composed of a range of different but linked habitats (some of which are designated in Annexe I of the Habitats Directive as habitat types in their own right).   They vary widely in both habitat and species diversity and type according to their geographic location, size, shape, form, geology and degree of exposure.      

Atlantic salt meadows

Salt marshes develop between mean low water and mean high water tide levels in areas of soft sediments (mud or fine sand) that are relatively sheltered from wave action.  Saltmarsh vegetation has to be salt-tolerant (halophytic) in order to survive being covered by the tides.  Obviously plants on the lower reaches of the saltmarsh which is covered by every incoming tide for some time need to be more salt-tolerant than those on the upper reaches which may only be splashed by waves during high spring tides.  This causes a zonation of different plant communities from the lower to the upper saltmarsh. The Carmarthen Bay and Estuaries cSAC has the largest area of saltmarsh wholly in Wales (over 2,000 ha). 

In the past much saltmarsh, especially the upper zones, has been lost to land reclamation (by a bank or wall cutting off the area from the sea).   This makes sites with complete  sequences of the different plant communities and transitions to other habitats such as sand dunes (as the Carmarthen Bay and Estuaries cSAC has) are particularly important.

Atlantic salt meadows occur in North Sea, English Channel and Atlantic  shores.  They form the middle and upper reaches of saltmarshes, where the vegetation is still covered by the tide but less often and for shorter times.   The vegetation is often species poor at the lower reaches of the saltmarsh.  An open sward of common saltmarsh-grass Puccinellia maritima may form here.  Further up the marsh plant diversity increases, the vegetation becomes herb dominated and red fescue Festuca rubra becomes more important.  The upper saltmarsh  plant communities vary considerably, especially where they grade into different habitats.  There is also regional variation around the UK.  Grazing by domestic animals is also important in determining the type of vegetation growing on the marshes.  Much of the saltmarsh within the cSAC is grazed by sheep, ponies and some cattle. 

Salicornia and other annuals colonising mud and sand.

This vegetation type is dominated by open stands of Glasswort Salicornia spp. (also known as marsh samphire).  It has low numbers of species but includes common saltmarsh grass Puccinellia maritima, common cord-grass Spartina anglica and sea aster Aster tripolium.  It is a pioneer vegetation type and forms mainly at the lower reaches of saltmarshes where it is frequently flooded by the tide (although it can also be found on open creek sides, depressions within saltmarsh and disturbed areas of upper saltmarsh).  There is little variation in this vegetation type although the density of the plants can vary (density may be lower on sandier substrates). 

The small, fleshy spikes of the glasswort  reduce water movement around them and so encouraged the deposition of   silt.  This raises the sediment level, causing it to be exposed to the air for longer and thus allowing less salt tolerant plants to grow.  The Salicornia thus acts as a precursor to more stable vegetation.   

This feature has been included in the Carmarthen Bay and Estuaries cSAC as the site has large areas of the habitat which form an integral part of a sequence of habitats from sand/mudflats through to more stable saltmarsh vegetation.

Mudflats and sandflats not covered by seawater at low tide.

Intertidal mudflat and sandflats are covered and uncovered by the tide.  They form a major component of both estuaries and embayments but also occur on the open coast.  These intertidal flats can range from the mobile, coarse-sand beaches of wave-exposed coasts to the stable, fine sediment mudflats of estuaries and embayments.  For ease of description they can be divided into three broad categories, although in practice they merge into each other.  The three categories are:

1. Clean sands.  These communities occur between the mid and low tide levels on clean, sandy beaches on the open coast and in bays where wave action or strong tidal streams prevent fine silt being deposited or where the supply of suspended silt is low.  As the sediment is mobile it can be abrasive.  This means that species that live in clean sands tend to be mobile and tough such as amphipod crustaceans (such as sandhoppers Bathyporeia spp.), some polychaete worms and bivalve molluscs.

2. Muddy sands.  These occur where there is enough shelter from wave action to allow the deposition of fine sediments but some water movement or lack of supply of silt leads to a more sandy substrate.  These conditions can occur at the mouths of estuaries or behind barrier islands.  A wide range of organisms can colonise these sediments, such as lugworms Arenicola marina, mussels Mytilus edulis, and eelgrasses Zostera spp..

3. Mudflats.  These form in the most sheltered areas of the coast, usually in estuaries which have large supplies of silt from the rivers draining into them.  Due to the sheltered nature of these areas the sediment is stable and dominated by polychaete worms and bivalve molluscs often with very high numbers of the snail Hydrobia ulvae.  The high biomass (weight of living things) in the sediment provides important feeding areas for waders and wildfowl.         

The Carmarthen Bay and Estuaries cSAC has a mixture of all three of these types.  The intertidal flats within the Carmarthen Bay and Estuaries cSAC provide an important feeding ground for many waders and wildfowl which is recognised by a section of it (The Burry Inlet) being designated as a Special Protection Area and Ramsar site.

Sandbanks which are slightly covered by sea water all the time.

This habitat type consists of soft sediment types that are permanently covered by shallow sea water (less than 20m deep).  SAC sites have been selected for this feature to cover the geographical and ecological range of variation of these categories

1. Gravelly and clean sands

2. Muddy sands

3. Eelgrass Zostera marina beds

4. Maerl beds

3 and 4 are distinct communites associated with shallow sublittoral sandy sediments and may support particularly valuable, diverse communities. 

The diversity and types of community associated with this habitat type varies according to sediment type, geographical location, exposure of the coast and the depth, turbidity and salinity of the surrounding water. 

Shallow sandy sediments are typically colonised by burrowing animals such as worms, crustaceans, bivalve molluscs, and echinoderms (starfish and urchins).  Prawns, crabs, snails and fish (such as sandeels - an important food for birds) may live on the surface.  Where sand and harder substrates mix there may be very rich communities.  Shallow sandy sediments can be important nursery areas for fish and feeding grounds for seabirds.  The Carmarthen Bay and Estuaries cSAC is an important nursery area for Bass Dicentrarchus labrax and feeding ground for the seaduck, common scoter Melanitta nigra.

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