||Range of conditions
The majority of exposed circalittoral rock
habitats occur on the open coast in full salinity, however some occur in the tide-swept
sounds of sealochs and in a few cases are subject to more variable salinities (e.g. Loch
||Extremely exposed, Very exposed, Exposed, Moderately exposed,
Sheltered, Very 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 will be
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
||Very strong, Strong, Moderately strong, Weak, Very weak
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 alter 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 will
result from a balance between their ability to withstand vigorous water movement, and
their need for water flow to assist their feeding processes. Exposed areas tend to be
dominated by cnidarians and massive sponges.
||Bedrock; stable boulders
Surface texture, erosion and rock
hardness are factors of marked 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 is 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 has been recorded (Knight-Jones &
Jones 1955). Mobile substrata under exposed conditions have a community characterised by
serpulid worms, barncales and bryozoan crusts (Hiscock 1981; Dipper 1983; Mitchell, Earll
& Dipper 1983; Bunker & Hiscock 1987; Howson 1988) rather than by the larger more
delicate species which feature on the adjacent bedrock.
||5-50 + m
||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 as the limit of kelp or dense algal growth.. 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 gulleys); 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. In exposed conditions temporarily suspended material, such as
coarse bottom material, may cause scour. Settlement of suspended material is not usually a
problem in exposed situations.
||Scour is a factor in more exposed areas where the rock
substratum is in proximity to sediment. Typically such situations are found where boulders
lie on a sandy bottom, or in the regions where the bedrock merges with the level seabed.