| 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. |
