| 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. |
