Salt marshes

Salt marshes, which are usually formed on the flat shores of protected bays and estuaries, have a vegetation of their own consisting of plants which rarely or never grow anywhere else because they are adapted to periodic immersion in salt water: many of them have succulent leaves. From the scattered glass-worts (marsh samphires) which extend farthest out towards the sea, and are covered by most high tides, there is a complete zoning of the vegetation with an increase in the number of species as one passes landwards to the highest levels of the marsh reached only by the highest spring tides, so that they are dry for most of the year and are covered by a continuous carpet of plants. These are nearly always used as sheep pasture-the French 'pres sales'. The turf is often gorgeous in June with the rose-pink flowers of thrift or sea pink, and later in the summer with the purple of sea lavender, along with several other characteristic plants. If the sea is kept out by dykes the salt is soon washed out of these pastures and they can be made into good agricultural land.

Salt marshes, like all maritime lands, are constantly changing under the direct influence of the sea. Where silt is continuously deposited by the tides the level of the soil gradually rises and the pioneer plants, such as glasswort, characteristic of the outermost mobile zone where the mud or sand is continually moved about by the tides, are gradually replaced by those of the more stable areas farther from the sea, while the pioneer plants may extend seawards on the bare mud. But the tidal currents and the river currents of estuaries often alter in direction and strength, so that different parts of the marsh, including the old stable marshland, may be washed away, exposing new bare soil, and this is recolonised by the pioneer plants unless the currents are too strong. Natural drainage channels are always formed in the marsh and these take the scour of the tides, so that they frequently alter their courses like the beds of rivers of strong current. Thus any extensive system of salt marshes forms a constantly changing system under the dynamic influence of the sea, and is of great interest to the physical geographer and the ecologist.

Nor are salt marshes by any means devoid of natural beauty. Their level stretches, covered by the sea at regular intervals, and then bared by the ebbing tide, show many shades of lovely colour illuminated by the changing lights. The ebb and flow of the highest spring tides which cover the whole marsh area are fascinating to watch, as the incoming sea rushes up the drainage channels and flows gently but irresistibly over the levels, while on the ebb the water gradually subsides on the flats, draining into the channels, which it scours out as it falls back seaward. For all these reasons a good selection of the best saltmarsh areas certainly ought to be preserved.

Mud flats

Mudflats are created by the deposition of fine silts and clays in sheltered low energy coastal environments such as estuaries, where they may form the largest part of the intertidal area. Mudflats play an important role in coastal defence, dissipating wave energy. They also have a high biological productivity with abundant invertebrates such as ragworms, lugworms, sandhoppers, cockles and Hydrobia snails that provide food for internationally important populations of migrant and wintering birds. Mudflats are also important fish nurseries for species such as plaice.

In the temperate zone most estuarine habitats are characterized by strong daily and seasonal fluctuations in oxygen saturation, temperature and salinity, and by pronounced tidal movements. Nevertheless, estuarine habitats are generally highly attractive to animal life: relatively large amounts of solar energy input to exposed mud flats, together with a high availability of nutrients through mineralization lead to considerable rates of primary production. In addition, estuaries with extensive mud flats and banks receive energy input in
the form of suspended organic material from the open sea by tidal currents.

Small and very small animals with a negligible biomass, but with a relatively high production, have probably been underestimated in the past.. This complex , referred to as 'small food web' - consists, in addition to bacteria, of microfauna, meiofauna, temporary meiofauna and small macrofauna.

Sandy beaches

To the casual observer, beaches may simply appear as barren stretches of beautiful sand, largely devoid of obvious signs of life. In reality, hundreds of species inhabit sandy beaches, and careful examination of sandy beaches reveals a hidden world of great animal diversity. We don't see this world because most organism are quite small (a centimetre long or less) and live buried in the sand.

Inhabitants of sandy beaches include representatives from all major groups in the food web such as decomposers (bacteria and fungi), plants (mostly small diatom algae), filter-feeding organisms (e.g. clams), scavengers (e.g. ghost crabs) and predators.

Sandy beaches provide habitat and support a great variety of living organisms. They are key ecosystems that link the sand dunes with the surf zone through a constant interchange of sand, organic matter and nutrients. The surf zones of beaches are an important nursery and recruitment area for fish that rely on the smaller invertebrates as a supply of food. For example, prey organisms (e.g. invertebrates) that live in the intertidal zone support fish populations and it is these fish that recreational fishermen target. Beaches are also home to a variety of shorebirds and the essential nesting habitat for turtles.

The constantly shifting sands of beaches do not support large plants with roots that are the base of all life on land. Yet, an amazing variety of life does flourish on our beaches. These animals are fuelled by inputs from the ocean that delivers plankton to the beaches' consumers. The sea also casts ashore larger dead organisms such as fish, jellyfish and other invertebrates which are eaten by the scavengers like ghost crabs and birds. Some beaches naturally accumulate considerable amounts of seaweed and seagrass on the upper shore near the dunes. This material often collects as a distinct "drift line". It may be unsightly to the casual observer, but such organic material is a vital food source and habitat to many animals.

Sand dunes

Sand dunes (Phots. 21, 22) are formed on parts of a coast where sand is deposited by the sea above the lowtide level, dried in the air during the period of ebb, and blown landwards. It accumulates against any obstruction and forms the well-known sand hillocks or dunes. Small dunes (foredunes) are formed first of all on the foreshore just above average high-tide level and are overrun by the highest spring tides. The sand heaps up round the isolated plants that grow on the foreshore, and one of the grasses (the sea twitch) is able to extend through the sand piled over it and thus to increase up to a point the size of these small dunes, which never attain any considerable height. Another kind of grass, however-the marram-which also grows on the sandy shore, though it cannot endure prolonged immersion in sea water, is able to push up almost indefinitely through superincumbent sand, and this creates the main mass of dunes which sometimes reach 100 ft. in height.

Marram grass, with its very long, narrow, rolled leaves, is the familiar dominant plant of practically all our coastal dunes. While the sand is still mobile the marram grows in tufts with bare sand between, which is colonised by plants like the ragwort and sea holly. The older dunes, however, farther from the sea, protected by the high mobile ones, and thus less exposed to sea winds, are colonised by mosses and lichens and many other plants, so that they become fixed by a continuous carpet of vegetation. When the fixed dunes are pastured they are predominantly grassy, but otherwise they may be colonised by shrubs or by heather, and the vegetation gradually loses its maritime character altogether.

Sand dune areas are of course the traditional sites of golf links, and this use has protected many of them from more devastating ' developments '. The ' greens ' and to some extent the ' fairways ' have their vegetation altered, but the `rough' remains in its natural state. Some sand-dune areas should, however, be kept untouched as nature reserves, for they not only bear a characteristic vegetation, but are the nesting places of many sea birds, among which the terns, laying their eggs among the foredunes, are conspicuous.

Like a salt marsh, a sand-dune area forms a changing system under the influence of a constant dynamic agent-in this case not the tide itself, but sand blown by the sea wind. Where the quantity of sand deposited offshore and blown landwards is very great it may become a threat to the country inland, and on some coasts and in some continental areas the so-called `travelling dunes' are a formidable menace. But few of our coastal dunes are dangerous in this way. Where the amount of loose sand is excessive, marram is often planted on the bare sand to check its movement.

Sand dunes are formed in suitable spots all round our shores. The western coasts are the richest and on these are found some of the most extensive dune areas, for example Braunton Burrows in North Devon, several in South Wales, Newborough Warren and others in Anglesey and the Lancashire dunes between Southport and Liverpool. A famous Scottish area is the Culbin Sands on the coast of Morayshire. This has been extensively planted with conifers, largely pine, which is much used to stabilise dunes and for the production of timber.

Several areas which are in no way threatening should however be kept untouched as nature reserves, for coastal dunes stand high among natural habitats both for their beauty and their physiographic and natural history interest.

Shingle beaches

The last important type of maritime habitat is the shingle beach (Phots. 23-25). The British areas of coastal shingle are in the aggregate far less extensive than salt marshes or sand dunes. They are formed where tidal currents drive shingle along the coast and the waves throw it up on the shore in the form of a fringing beach, or drive it across a bay or the mouth of an estuary as a shingle spit or bar. In some few places wide areas of shingle are formed by a number of successive fringing beaches laid alongside one another. Dungeness is the most extensive example in Britain.

Plants cannot of course maintain themselves in shingle which is constantly and often violently moved by the tide, nor can they root and feed themselves between bare pebbles, but there is always a certain amount of sand mixed with the stones, and the roots depend upon this and especially upon the considerable amount of humus derived from decayed seaweed, driftwood and other organic debris thrown up by the sea beyond high tide mark. The humus is washed down between the stones by rain so that a considerable vegetation can develop on the shingle. On the back of a fringing beach or spit, just beyond the 'storm crest'-the highest ridge of shingle thrown up when a high spring tide coincides with an onshore gale-many plants settle down, and of these some are characteristic of this situation, though several are also found on sand dunes. Of the shingle beach plants proper the purple herb-robert, the sea nightshade, and the sea pea are the most striking, while the yellow-horned poppy and the sea tampion occur alike on dunes and shingle. A good many non-maritime plants also colonise the shingle when it begins to escape the direct influence of the sea. At Dungeness, for instance, land scrub-blackthorn, gorse, etc.-develops on the wide tracts of shingle at some distance from the shore, and other areas become grass-covered.

Shingle spits are fairly common on the south and east coasts of England, and the history of their development is of great interest. A spit is formed where a straight stretch of coast is broken by a bay or estuary, and the shingle, driven along the shore, continues its course across the mouth of the inlet. The tip of the growing spit is often driven landwards by a violent storm, forming an incurved ' hook'. The growth of the spit is afterwards continued in the same direction as before and later the new tip is again driven in. In this way a number of lateral branches of the main spit may be formed and a complex system of shingle banks created. The spits commonly enclose salt marshes, which develop on their sheltered inner sides; and on the top of the shingle, where it is at all extensive, sand dunes may develop if sand is being deposited in quantity offshore, so that it can be blown on to the shingle.

In this way all three types of maritime habitat-salt marsh, sand dune and shingle-are often closely associated in one complex, and the preservation of an example of all three can be secured in one nature reserve.

Notable examples are Blakeney Point and Scolt Head Island, on the north coast of Norfolk. The longest English shingle spit is on the Suffolk coast, where a great and long-continued drift of shingle southwards has crossed the old estuaries of both the Alde and the Butley rivers and driven their now combined outlet to the sea some 12 miles south of the original mouth of the Alde. The most famous English shingle beach is the Chesil Bank in Dorset (Phots. 23, 24, 25), extending south-eastwards from Burton Bradstock to Portland Bill, for some 6 miles as a fringing beach (Phot. 24) and then for 20 miles as a bar (Phots. 23, 25) enclosing the waters of `the Fleet', a lagoon between the bar and the mainland. This is a magnificent lofty beach with many interesting structural features and a well-developed flora, including an almost continuous fringe along the Fleet of a Mediterranean shrub (Suaeda fruticosa), which is confined in England to a few places on the south and east coasts.