||Range of conditions
||Full, Variable, Reduced / low
||Sheltered, Very sheltered, Extremely sheltered
||Sandy mud, mud
||Strandline, Upper shore, Mid shore, Lower shore
||Supralittoral, Littoral fringe, Eulittoral
||Littoral mudflats are predominantly clay (particles
<4 m), silt (4-63 m) and to a lesser extent very find sand (63-125 m).
settling velocity of particles is dependent on particle size and water characteristics
such that clay and silt particles are unlikely to settle within one tidal cycle.
The type, direction and speed of the currents and the size of the particles control
sediment deposition within an area. Fine-grained material such as clay and silt will
follow the residual waterflow, although there may be deposition at periods of slack water.
||Clays can have porositys ranging from 65-82% and silts
45-88% (Taylor Smith & Li 1966). However, in extreme cases a mudflat that is composed
largely of clay can become sufficiently compacted to support sessile fauna and even
rock-borers such as the burrowing bivalve piddock Pholas (Eltringham 1971).
||The porosity and compaction of the sediment, the shore slope
and the potential for draining influence the water content of mudflats. Mudflats may be
extensive yet retain water at low tide as a result of their shallow gradient and the
capillary attraction of closely-packed particles (Gray 1981). The sediments may be
thixotropic due to the high water content (Chapman 1949), thus allowing easier burrowing
by infauna applying pressure to the sediment which becomes softer and easier to penetrate.
||Intertidal mudflats contain a high proportion of organic
matter which is deposited and accumulates in low energy areas due to its small and low
specific gravity. Allochthonous organic material is derived from both anthropogenic
sources (effluent, run-off) and natural sources (settlement of plankton, detritus).
Autochthonous organic material on these sediment areas is restricted to benthic microalgae
(microphytobenthos) such as diatoms and euglenoids and heterotrophic microorganism
production, although mats of opportunistic green macroalgae such as Enteromorpha
and Ulva will also develop. The organic matter (measured as organic carbon and
nitrogen) is degraded by the micro-organisms and the nutrients recycled (Newell 1965;
Trimmer et al. 1998). In addition, the high surface area-to volume ratio of fine
particles acts as a surface for the development of microfloral populations. These features
coupled with poor oxygenation of muds and hence low degradation rates, lead to an
accumulation of organic matter.
||Oxygen content is a function of the degree of oxygenation
(aeration) and the inherent oxygen demand of organic matter. Mud tends to have lower
oxygen levels than other sediment types because their lower permeability leads to the
trapping of detritus which, together with the large surface area for microbial
colonisation, leads to higher oxygen uptake (Eagle 1983). Much of the organic detritus
therefore undergoes anaerobic degradation, with hydrogen sulphide, methane or ammonia
produced, as well as dissolved organic carbon compounds which can be utilised by aerobic
micro-organisms living on the surface (McLusky 1989; Libes 1993).
||It has been calculated that the biomass of bacteria within
mudflats may be of the same order of magnitude as the biomass of animals living in the
sediment. Breakdown of organic matter to sulphides and sulphates by bacteria forms the
sulphur cycle, which determines the redox potential and pH of the sediment.