||Range of conditions
The majority of moderately exposed circalittoral rock
habitats occur on the open coast in full salinity.
||Moderately exposed, Sheltered
Water movement is the prime
factor influencing community composition. Wave action generates extreme forces, and is
basically a result of wind blowing across the sea and transferring energy to the sea
surface. Wave action is modified by local topography and the severity of wave effects
decrease with depth. Under gale conditions the bottom water velocity may be > 200
cm.sec-1 at 20 m, but reduced to about 60 cm.sec-1 at 40 m and 9
cm.sec-1 at 80 m (Hiscock 1983).
||Moderately strong, Weak, Very weak
Tidal streams flow to
and fro with the tidal cycle, and they do not attenuate with depth as rapidly as does wave
action. The presence or absence of water movement will alters the balance of competition
between species, which might be otherwise able to survive across a wide range of exposure.
The end result is that there are very different circalittoral biotopes in different
conditions of current exposure. The distribution of species results from a balance between
their ability to withstand vigorous water movement, and their need for water flow to
assist their feeding processes.
||Bedrock; stable boulders and cobbles
erosion and rock hardness are factors of obvious relevance to circalittoral communities.
Substratum stability is determined by whether it is comprised of bedrock, or of loose
boulders or stones. The mobility of boulders and stones will be a function of wave
exposure, and mobility of the substratum will selectively impact faunal turf species.
Marked differences between the communities of bedrock and adjacent loose rocks have been
recorded (Knight-Jones & Jones 1955). Mobile substrata under exposed conditions have a
community characterised by serpulid worms, barnacles and bryozoan crusts (Howson 1988;
Bunker & Hiscock 1987; Dipper 1983; Mitchell, Earll & Dipper 1983, Hiscock 1981)
rather than by the larger more delicate species which feature on the adjacent bedrock.
||Localised short-term fluctuations in seawater temperature,
resulting from heat loss or gain to the air or the substratum, can occur in the shallow
surface layer in inshore water. Circalittoral faunal turf communities are largely
insulated from such transient influences by their depth and in many cases also by their
prevalence in high-energy systems.
||Light is the environmental factor which determines the depth
distribution of the circalittoral the decrease of light with depth defines the
upper limit of the zone. In areas where enough incident light reaches the seabed rocky
habitats the community tends to be dominated by large macroalgae in what is defined as the
infralittoral zone. When light levels decline with depth there is a progressive shift to
faunal-dominated communities. Areas of the infralittoral dominated by animal biotopes
occur as a result of steep slopes, intense grazing, and sometimes extreme physical
conditions (such as surge gullies), however they are very much the exception.
||The slope of the rock influences faunal turf communities as
it affects the amount of incident light, and consequently the abundance of algal growth.
||Transparency and water clarity are affected by dissolved
material and suspended particles in the water, and are important because they influence
the penetration of light.