Colne Estuary Habitats

The Colne estuary is a comparatively short and branching estuary, with five tidal arms which flow into the main river channel. The estuary has a narrow intertidal zone predominantly composed of flats of fine silt with mudflat communities typical of south-eastern estuaries.

The estuary is of international importance for wintering Brent Geese and Black-tailed Godwit and of national importance for breeding Little Terns and five other species of wintering waders and wildfowl.

The variety of habitats which include mudflat, saltmarsh, grazing marsh, sand and shingle spits, disused gravel pits and reedbeds, support outstanding assemblages of invertebrates and plants.

Tidal mud flats 30%
Salt marshes 25%
Freshwater marshes 20%
Estuarine waters 19%
Sand / shingle shores (including dune systems) 3%
Coastal brackish / saline lagoons 2%
Marine beds (e.g. sea grass beds) 1%

Saltmarsh

Saltmarsh has colonised a large proportion of the estuary at Geedon Saltings, Colne Point and the Strood. The majority of this is high-level marsh dominated by saltmarsh grass Puccinellia maritima, sea purslane Atriplex portulacoides and annual seablite Suaeda maritima while the creek edges and disused oyster pits have been colonised by glasswort Salicornia spp, sea aster Aster tripolium, and cord grass Spartina spp. There are extensive saltpans on Geedon Saltings and Colne Point where there is a shorter sward of saltmarsh grass, thrift Armeria maritima and common sea-lavender Limonium vulgare.

Nationally uncommon species such as golden samphire Inula crithmoides and shrubby sea blite Suaeda vera occur frequently in the upper marsh and at the foot of the sea-walls. Shrubby sea blite is particularly extensive at Colne Point where there is a transition from saltmarsh to sand dune and shingle. This transition habitat is also important for the nationally uncommon rock sea-lavender Limonium binervosum and is one of the few East Anglian sites for sea heath Frankenia laevis.

Salt marsh is a type of marsh that is a transitional intertidal area between land and salty or brackish water (e.g. estuaries). It is dominated by halophytic (salt tolerant) herbaceous plants. Salt marshes are one of the most biologically productive habitats on the planet. The daily tidal surges bring in nutrients, which tend to settle in roots of the plants within the salt marsh. The natural chemical activity of salty (or brackish) water and the tendency of algae to bloom in the shallow unshaded water also allows for high biodiversity.

Salt marshes provide a benefit by acting as buffers to severe weather and tides, providing a degree of coastal protection. In the past, substantial areas of saltmarsh have been reclaimed as agricultural land and for urban development, but they are now accorded a high level of protection under the Habitats Directive. There is growing interest in restoring salt marshes, through managed realignment (e.g. Abbotts Hall, Wallasea Island).

For more information about Saltmarsh as a UK BAP priority habitat click here.

Mudflat

The Colne Estuary has a narrow intertidal zone predominantly composed of flats of fine silt with mudflat communities typical of south-eastern estuaries. The fauna is dominated by Hydrobia ulvae snails with Macoma balthica & Scrobicularia plana bi-valves, Hediste diversicolor, and Nephtys hombergii worms. Towards the mouth of the estuary the substratum becomes more sandy; seagrassess such as Zostera noltei and Zostera marina have been recorded at Sandy Point.

Mudflats are coastal wetlands that form when mud is deposited by tides or rivers. They are found in sheltered areas such as bays and estuaries. Most of the sediment within a mudflat is within the intertidal zone resulting from deposition of estuarine silts and clays, and thus the flat is submerged and exposed approximately twice daily at low tide.

Mudflats are important regions for wildlife, supporting large populations, although levels of biodiversity are not particularly high. They are often of particular importance to migratory birds. In the United Kingdom mudflats have been classified as a Biodiversity Action Plan priority habitat.

The maintenance of mudflats is important in preventing coastal erosion. However, mudflats worldwide are under threat from predicted sea level rises, land claims for development, dredging for navigation purposes, and  pollution.

For more information about Mudflats as a UK BAP priority habitat click here.

 

Freshwater grazing marsh

Coastal grazing marsh is a specific term applied to land derived from the enclosure of salt marsh. Coastal grazing marsh is recognised as a distinct habitat type in Great Britain. These areas represent some of the most recent (within the last 200-300 years) enclosures.

It is defined by the presence of permanent and semi-permanent grassland, drainage ditches and enclosing earth dykes. Features of the original marsh are present including old creek lines. Saline waters derive from seepage zones through the sea walls and intrusion of sea water along channels which may remain partially open to the tide. As with other wet grasslands the wildlife interest has developed alongside the traditional agricultural use of the land for grazing or hay-making.

The unimproved permanent pasture, when used for low intensity grazing, develops a vegetation structure attractive to nesting birds. These include redshank, lapwing, snipe, curlew and oystercatcher which also breed on salt marsh. In the winter surface flooding of parts of these same areas attract wintering wildfowl such as teal, wigeon and waders. The presence of short grassland also provides grazing for waterfowl, including the brent goose.

Rare species of plants are often found in association with the pasture and the brackish water drainage ditches. The brackish water ditches are particularly important for a number of rare invertebrates. Sites may contain seasonal water-filled hollows and permanent ponds with emergent swamp communities, but not extensive areas of tall fen species like reeds.

For more information about freshwater and coastal grazing marsh as a UK BAP priority habitat click here.

 

Marine

Sheltered muddy gravel habitats occur principally in estuaries, in areas protected from wave action and strong tidal streams. In fully marine conditions on the lower shore this habitat can be extremely species-rich because the complex nature of the substratum supports a high diversity of both infauna and epifauna. Polychaete worms and bivalve molluscs are normally dominant, but representatives of most marine phyla can be present. The fauna is often characterised by a large range in body size. As one moves into an estuary, with a consequent reduction in salinity, there is a marked reduction in species richness. Low salinity (mid to upper estuarine) muddy gravels have a lower, but distinctive, species diversity.

Sublittoral sand and gravel habitats occur in a wide variety of environments, from sheltered estuaries to highly exposed conditions (open coast). The particle structure of these habitats ranges from mainly sand, through various combinations of sand and gravel, to mainly gravel. While very large areas of seabed are covered by sand and gravel in various mixes, much of this area is covered by only very thin deposits over bedrock, glacial drift or mud. The strength of tidal currents and exposure to wave action are important determinants of the topography and stability of sand and gravel habitats.

The diversity of flora and fauna living within the biotopes varies according to the level of environmental stress to which they are exposed. Sand and gravel habitats that are exposed to variable salinity in the mid- and upper regions of estuaries, and those exposed to strong tidal currents or wave action, have low diversity and are inhabited by robust, errant fauna specific to the habitat such as small polychaetes, small or rapidly burrowing bivalves and amphipods. The epifauna in these habitats tends to be dominated by mobile predatory species. Upper estuarine mobile sands, subject to very low fluctuating salinity, are species poor. This habitat is characterised by mysid shrimp (Neomysis integer) and amphipods (Gammarus spp).

The epifauna is characterised by mobile predators such as crabs (Carcinus maenas and Liocarcinus spp), hermit crabs (Pagurus bernhardus), and whelks (Buccinum undatum).

For more information about sub-littoral gravels and sands as a UK BAP priority habitat click here.

 

 

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