Offshore sands and gravel
What are they?
Sublittoral sand and gravel sediments are the most common habitats found below the level of the lowest low tide around the coast of the UK. The sands and gravels found to the west of the UK (English Channel and Irish Sea) are largely shell derived, whereas those from the North Sea are largely formed from rock material. Both inshore (defined as extending to six nautical miles), and offshore (six nautical miles to the limit of UK waters) environments are encompassed by these proposals.
Sublittoral sand and gravel habitats occur in a wide variety of environments, from sheltered (sea lochs, enclosed bays and 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 varies according to the level of environmental stress to which they are exposed:
Sand and gravel habitatsthat 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 mysids (Neomysis integer) and amphipods (Gammarus spp).
Coarse sand sedimentcan occur in sand-wave formations in shallow water, wave exposed and tide-swept coasts.The infauna in this type of habitat is highly impoverished and is typified by small opportunistic capitellid and spionid polychaetes and isopods (Pontocrates arenarius, Haustorius arenarius and Eurydice pulchra) that are adapted to living in a highly perturbed environment. The epifauna is characterised by mobile predators such as crabs (Carcinus maenas and Liocarcinusspp), hermit crabs (Pagurus bernhardus), whelks (Buccinum undatum), and occasionally sand eels (Ammodytesspp). Similar habitats also occur in estuaries where the marine fauna is replaced with a sparse low salinity tolerant fauna (Forth and Humber Estuaries, Solway Firth).
Well sorted medium and fine sandson exposed coasts subjected to frequent wave action and variable tidal currents are typified by errant polychaetes such as Nephtys cirrosaand isopods such as Bathyporeia spp (common in full salinity areas of many estuaries). A low salinity variant of this habitat occurs in the Humber and Severn Estuaries.
Loose, coarse sand habitatsfully exposed to wave action and swept by strong tidal streams are comparative with the 'Shallow Venus Community', the 'Boreal Off-shore Sand Association' and the 'Goniadella-SpisulaAssociation' defined in past studies. This habitat is dominated by small or highly mobile polychaetes, thick shelled and rapidly burrowing bivalves (Spisula elliptica and S. subtruncata) and mobile amphipods that are adapted to periodic disturbance. It is a common habitat with examples found from Shetland to the Scilly Isles.
A close variant of this community occurs in fine compacted sands with moderate exposure and weak tidal currents. This habitat is characterised by the thin-shelled bivalve Fabulina fabula, and is found in the Irish Sea, north-east coast of England and in numerous Scottish sea lochs.
Sand mixed with cobbles and pebblesthat is exposed to strong tidal streams and sand scour is characterised by conspicuous hydroids (Sertularia cupressina and Hydrallmania falcata) and bryozoans (Flustra foliacea andAlcyonidium diaphanum). These fauna increase the structural complexity of this habitat and may provide an important microhabitat for smaller fauna such as amphipods and shrimps. Examples of the habitat are to be found in Shapinsay Sound, Cromarty Firth, Lowestoft, Thames, Thanet, Menai Strait, Lough Foyle and in numerous Scottish sea lochs.
In contrast, those biotopes found in full salinity in sheltered or deeper watersthat are less perturbed by natural disturbance are among the most diverse marine habitats with a wide range of anemones, polychaetes, bivalves, amphipods and both mobile and sessile epifauna. Clean stone gravel habitats are characterised by the sea anemones Halcampa chrysanthellum and Edwardsia timida, associated with hydroid/bryozoan turfs and red seaweeds. This habitat type has limited recorded distribution: Loch Creran, Loch Eynort (Skye), Church Bay (Rathlin Island) and Strangford Narrows.
Shallow areas with coarse sandswept by tidal currents but sheltered from wave exposure may develop dense beds of the polychaete Lanice conchilega. Dense beds of polychaete tubes reduce near-bed currents and significantly increase sediment stability. Examples are to be found in Outer Hebrides lagoons, Skye and sea lochs.
Circalittoral gravels, sands and shell gravelare split into three different biotopes and have communities of high diversity. These habitats are dominated by thick-shelled bivalve and echinoderms species, (eg Pecten maximus, Circomphalus casina, Ensis arcuatus andClausinella fasciata), sessile sea cucumbers (Neopentadactyla mixta), and sea urchins (Psammechinus miliaris and Spatangus purpureus). These biotopes have been described by previous workers as the 'Boreal Off-Shore Gravel Association' and the 'Deep Venus Community' and can be found in Shetland, the western coasts, Irish Sea and English Channel.
Many of the inshore habitats are important nursery grounds for juvenile commercial species such as flatfishes and bass. Offshore, sand and gravel habitats support internationally important fish and shellfish fisheries.
Sand and gravel habitats. These are subjected to a variety of anthropogenic factors including the influence of pollutants in riverine discharge, and physical disturbance by fishing and aggregate dredging activities. The latter two probably have the greatest influence on the organisms that inhabit sand and gravel substrata. Most flatfish fisheries are found in areas of sandy seabed and are subjected to intensive perturbation by bottom fishing gears (such as beam trawling) in the southern North Sea and English Channel. Gravel substrata are disturbed by scallop dredging, particularly in the English Channel and northern Irish Sea. Gravel habitats are severely modified by aggregate extraction in licensed areas off the east and south-east coast of England. These disturbances are less prevalent north of the Firth of Forth, on the west coast of Scotland, and in large parts of the Irish Sea.
Fishing. Many species inhabiting highly perturbed and mobile sediments are relatively unaffected by fishing activities or other anthropogenic physical disturbance. However, large-bodied, slow-growing fauna such as bivalves are sensitive to fishing disturbances and their populations may be slow to recover. Biogenic reefs and sedentary worm beds may be particularly vulnerable to trawling activity. Some of the bivalve species found in these habitats, such as Pecten maximus, are subject to significant fishing effort. Other species, such as Paphia rhomboides, Glycymeris glycymeris, Chamelea gallina, and Ensis spp are subject only to occasional fishing effort. Most of these are exported to continental Europe for human consumption. Fishing may alter the trophic interactions within these habitats by removing predators and competitors. However, the removal of some species may not necessarily adversely affect the ecological functioning of the community.
Aggregate extraction. This is restricted to smaller and strictly defined areas. However, in some places within the licensed dredged areas, the impact on the seabed can be greater per unit area than bottom fishing as both the substrata and fauna are removed, which prolongs the recovery of the habitat and benthic community. Such major impacts can be limited, however, as some areas within a licensed area are commercially unattractive because the aggregrate resource is too thin. Once an area has been dredged and aggregate removed, the operator generally moves on and recovery begins. Areas that are heavily fished, however, may never fully recover because the seabed is disturbed before recovery has taken place.
Other physical disturbances. These include land claim, construction of marinas and slipways, the widening and dredging of channels, pipe and cable laying and the construction of sea defences. These activities can alter tidal flow regimes and wave exposure, or result in deposition of sediments that influence the structure of sedimentary habitats.
Pollution. Organic pollution from sewage discharge and aquaculture activities leads to anoxic conditions and a decrease in benthic diversity. Pollution is also caused by persistent bio-accumulating chemicals (eg polychlorinated biphenylsand tri-butyl tin), heavy metals and other chemicals. These pollutants have led to decreases in the populations of common whelks in the North Sea and cause DNA breakdown in some marine organisms
Oil exploration, leakages and shipping accidents lead to localised pollution of sediment organisms.
Vision for sublittoral sands and gravels
The South East Biodiversity Forum’s vision for this habitat is that there should be:
- No further loss of existing habitat
- Good management, including visitor management, on all extant sites
- No damage to site integrity from activities arising outside the sites, eg. inadequately managed public access
- Re-creation of sand dunes on appropriate sites to restore some past losses, including the linking up of fragmented sites
- Greater public appreciation of sand dunes and their specialist wildlife, including greater awareness of the impacts of human pressures, such as dog-walking, mountain-biking, dumping of waste
- Creation of alternative green space around important sand dunes under pressure for increasing new housing