mud flats

mud flats Mud flats are gently sloping or flat expanses of fine-grained sediment. They form in sheltered environments around the coast and in estuaries, where deposition is favoured by low-energy conditions. Mud flats are made up of fine particles of mineral and organic origin, generally in the silt size range (0.002 to 0.06 mm) or clay size range (less than 0.002 mm). The current velocities required for mud deposition are extremely low, and intertidal mud flats characteristically accumulate sediment when current velocities fall to zero during the slack water period at high tide. Muds are cohesive sediments, which despite their small particle size have a large surface area relative to mass. They are rich in clay minerals, which have an interlayered crystal structure and (normally) a net negative surface charge. The cohesive nature of mud-flat sediments is a result of interparticle attraction between these clay minerals. Particles cohere (or flocculation occurs) when the repulsive surface charges on the clay minerals are suppressed in a weak electrolyte such as sea water. Flocculation is reinforced by organic cohesion, where particles are bound by mucus secretions or biogenic pelletization. The cohesive nature of mud means that after deposition it is relatively difficult to erode, aiding the build-up of sediment on the mud flats.

Owing to the fine nature of mud-flat sediments, the spaces between the individual sediment particles are extremely small, maximizing water retention as the tide retreats and minimizing the penetration of oxygen into the sediment. The high organic-matter content of mud-flat areas (as much as 5 per cent of the sediment may be organic carbon) provides a rich food source both for burrowing and grazing animals and for bacteria which consume the organic material. A consequence of limited oxygen penetration into the sediment is that the bacteria present quickly use up the available oxygen to break down the organic material. Under these conditions, breakdown of organic matter continues using oxygen from other sources, particularly the sulphate ion (SO₄²⁻). It is this sulphate reduction which gives mud flats their characteristic ‘rotten eggs’ smell, produced when SO₄²⁻ is reduced (or used as an electron acceptor) leading to formation of hydrogen sulphide, H₂S, and HS⁻. In organic-rich, extremely fine-grained mud-flat areas, sulphate reduction may occur only a few millimetres below the sediment surface. This produces a clear colour change in the sediment from light brown to grey-black (the grey-black coloration being caused by precipitation of iron sulphides).

In comparison with the salt-marsh environments that may form on upper mud-flat areas, the mud flat proper is poorly vegetated. Seaweeds may be found where solitary pebbles or beds of molluscs provide a stable anchoring point, but only a few flowering plants have adapted to truly marine conditions, all of which are in the genus Zostera (Eel Grass). This genus was formerly widespread on mud flats along the lower shore and below the low tide mark. In the 1930s, however, the Zostera beds were almost completely wiped out by disease, although there has since been a degree of recovery.

The mud-flat fauna is governed to some extent by sediment grain size. Areas exhibiting mixed sand and mud constitute a relatively stable substrate in which typical sand-living animals, such as sand gapers, lugworms, and razor shells, dwell. Mud-flat areas proper are characterized by an unstable substrate in which oxygen-free conditions and the fine nature of the sediment combine to clog the breathing and feeding mechanisms of many species. As a result, relatively few species have adapted to mud-flat conditions, but those animals that are present are often found in vast numbers because of the rich supply of food in the highly organic sediment. Typical mud-flat fauna in northern Europe (Fig. 1) include the deposit-feeding bivalve Scrobicularia plana, the snail Hydrobia (Peringia) ulvae, the amphipod Corophium volutator, and the ragworm Nereis diversicolor. Large mussel beds are also common in many areas. Burrowing animals may act to increase or decrease the stability of mud-flat areas; mucus secretions may bind the sediment and make it less susceptible to erosion, whereas sediment stirring (bioturbation) may mix and re-suspend sediment. Mud-flat fauna provide a major food source for waders and wildfowl and, as such, these environments may be host to thousands of overwintering birds. The ecological importance of mud flats has been recognized by the designation of some as international sites, but despite their obvious conservation value, they remain threatenedin many areas by pollution, over-exploitation, and land reclamation.

Andrew B. Cundy

Bibliography

Davis, R. A. Jnr. (ed.) (1985) Coastal sedimentary evironments (2nd edn). Springer-Verlag, Berlin.