Habitat requirements

Habitat factor Range of conditions
Salinity Full, Variable, Reduced / low
Wave exposure Very exposed, Exposed, Moderately exposed, Sheltered, Very sheltered
Substratum Gravel, sand

Infralittoral mobile sandbanks contain all grades of sand (63µm-1mm) with a very low silt and clay content. One of the features of such a mixture of particle sizes is their low sorting coefficient. Small particles occupy the spaces between larger grains and thus reduce pore space. Another feature of subtidal sandbanks is they may have a highly dynamic nature and instability resulting from the inability of material to form cohesive clumps.

Depth band 0-20m
Zone Infralittoral
Vertical elevation Infralittoral gravels and sands occur within the photic zone and will therefore sustain many primary producers (Hiscock 1983). Any increase in depth or turbidity of the water will affect the light penetration and thus the primary producers; in the case of the biotope complexes covered here, the primary producers are benthic microalgae. The quality of light reaching such sandbanks will determine the type of microalgae colonising the sediment. In shallow or constricted areas the water above the banks may be very turbid (Carter 1988) thus limiting primary production.

Any change in water depth would change the characteristics of the sandbank. If water depth were to decrease, the sandbank may become exposed on low spring tides, which would decrease survival of subtidal fauna that cannot withstand exposure. The depth of the sandbank would also affect predator populations of birds, which are restricted to certain diving depths.

Porosity Particle size, its mixture and compaction influence the permeability or percolation rate (Pethick 1984) especially those with a mixture of particles. Infralittoral sandbanks tend to have a high porosity. The instability of infralittoral sandbanks and the inability of the material to form cohesive clumps prevents the colonisation of vegetation but allows the development of interstitial populations of organisms.
Organic content Infralittoral sandbanks typically have low levels of organic matter and are well oxygenated in the surface layers (Eagle 1973), the organic matter derived from decaying seaweed, the faeces and remains of animals. The mobile nature of this substrate produces a deeper anaerobic layer (>15 cm) and any organic matter incorporated into the sediment is degraded rapidly. High-energy areas have a low carbon to nitrogen ratio due to the low organic content, reduced productivity and rapid degradation of labile organic material.
Oxygen content Oxygen content is a function of the degree of oxygenation (aeration) and the inherent oxygen demand of organic matter. As infralittoral sand has a low organic content, they are usually sufficiently oxygenated by seawater which may percolate to several metres (Eagle 1983).
Microbial activity Microbial activity is low in areas of higher energy as there is limited organic detritus available for bacterial degradation coupled with the particles’ comparatively low surface area to volume ratio that provides a surface for microbial populations.

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