Habitat requirements

Habitat factor Range of conditions
Salinity Full, Variable

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 Etive, Scotland).

Wave exposure 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 1983).

Tidal streams Very strong, Strong, 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 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.

Substratum 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.

Depth band 5-50 + m
Zone Circalittoral
Temperature 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 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.
Slope The slope of the rock influences faunal turf communities as it affects the amount of incident light, and consequently the abundance of algal growth.
Water quality 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 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.

Next Section                     References