Habitat requirements

Habitat factor Range of conditions
Salinity Full, Variable
Wave exposure Sheltered, Very sheltered
Substratum Muddy sand

Littoral muddy sands are predominantly sand (particles 63-125 Ám) and to a lesser extent a mud fraction (4-63 Ám).

The settling velocity of particles is dependent on particle size and water characteristics such that sands and coarse materials settle rapidly and particles >15 mm will settle out within one tidal cycle (King 1975). 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. Coarser-grained material will travel along the bed in the direction of the maximum current and will be affected most by high velocities (Postma 1967).

Height band Strandline, Upper shore, Mid shore, Lower shore
Zone Supralittoral, Littoral fringe, Eulittoral
Porosity Porosity denotes the amount of pore space in a sediment and is related to the permeability of a sediment. Particle size, its mixture and compaction influence the permeability (Pethick 1984) especially those with a mixture of particles. Porosities in different sized material may be similar (Taylor Smith & Li 1966) due to interaction between grain shape, the degree of sorting, the length of time since deposition and therefore the degree of settling and compaction.
Water content The porosity and compaction of the sediment, the shore slope and the potential for draining influence the water content of mud and sandflats. Muddy sands retain water at low tide as a result of their shallow gradient and the capillary attraction of closely-packed particles (Gray 1981). However, muddy sands tend to be more freely-draining than mud alone owing to the greater average particle size.
Organic content Intertidal muddy sands 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 spp. and Ulva spp. will also develop. The organic matter (measured as organic carbon and nitrogen) is degraded by the microorganisms 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 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).

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