Habitat requirements

Habitat factor Range of conditions
Salinity Full, Variable, Reduced / low
Wave exposure Sheltered, Very sheltered, Extremely sheltered
Substratum Sandy mud, mud
Height band Strandline, Upper shore, Mid shore, Lower shore
Zone Supralittoral, Littoral fringe, Eulittoral
Substratum Littoral mudflats are predominantly clay (particles <4 m), silt (4-63 m) and to a lesser extent very find sand (63-125 m).

The 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.

Porosity Clays can have porosity’s 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).
Water content 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.
Organic content 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 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).
Microbial activity 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.

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