||Range of conditions
||Sheltered, Very sheltered
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).
||Strandline, Upper shore, Mid shore, Lower shore
||Supralittoral, Littoral fringe, Eulittoral
||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.
||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.
||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 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).