Habitat requirements

Habitat factor Range of conditions
Salinity Full , variable, reduced

As a result of the depths at which they occur reduced salinity is rarely a significant factor. Nevertheless reduced salinities do occur in sealochs at depths in excess of 30m.

Wave exposure Sheltered, Very sheltered
Tidal streams 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. Sheltered areas tend to be dominated by ascidians and more delicate sponges.

Substratum Bedrock; boulders and cobbles; mixed substrata

Large boulders which are not regularly displaced will provide a variety of cryptic environments on their undersides.

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. Seasonal shallow thermoclines may form, particularly in sheltered areas such as sealochs, and extend down to 15 m. Some animals such as the brachiopods Neocrania and Terebratulina seem restricted to below this thermocline (Hiscock 1985).
Light Light is the environmental factor which basically determines the upper depth limit 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 the rock substratum community tends to be dominated by large macroalgae creating the infralittoral zone.
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. Suspended material in the water can settle out of the water column, and can affect both the settlement and survival of circalittoral rock communities. This is usually only a problem in sheltered conditions where water movement is minimal.

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