We have seen that light basically determines the depth distribution of the circalittoral communities. Within that depth range, however, water movement is the prime factor influencing community composition. This is recognised within the JNCC biotope classification, where the first division of Circalittoral Rock Biotopes is into the three categories of 'exposed', 'moderately exposed', and ‘sheltered’ rock.

In the coastal waters, water movement arises predominantly from two sources - the action of waves, and currents generated by tides, and 'exposure' in the JNCC classification refers to either or both. In general the two often go hand in hand, with wave-swept headlands at the same time being in the path of the strongest tidal currents. But this is not always so, and there are areas sheltered from waves but receiving very fast tidal currents, as in the entrances of sea lochs and of rias in south west Britain. There are also areas of strong wave action but little current flow. This diversity is reflected in the number of biotopes listed.

Wave action: this generates extreme forces, and is basically a result of wind blowing across the sea and transferring energy to the sea surface. It depends on three factors - the force of the wind, the duration it blows for, and the distance over which it blows (the fetch) - as any or all of these increase, so does the wave severity. Wave action will also be modified by local topography. A very comprehensive account of the theory of waves is given in Denny (1988), and only a few points need to be made here. The water movement generated by wave action is turbulent, and very difficult to measure underwater in the field. 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). With increasing depth the water movement at the bottom tends to become oscillatory rather than multi-directional (Hiscock, 1983).

Tidal currents: these are easier to predict and measure than wave action. They normally flow to and fro with the tidal cycle, and they do not attenuate with depth as rapidly as does wave action. They may reach velocities up to 8 knots in straits between land masses (e.g. the Corryvreckan between Jura and Scarba in the Hebrides), or in the entrances to sea loughs (e.g. Strangford Lough in Northern Ireland). These extreme currents generate severe overfalls and whirlpools. A short general account is found in Hiscock (1983).

Water movement will affect species (and communities) in various ways, some beneficial, others detrimental. Beneficial effects include:

• Provision of suspended food supply for filter feeders.
• Prevents clogging of gills or other organs by settlement of silt.
• Ensures a high dissolved oxygen content.

• Physical damage or dislodgement of organisms.
• Restriction of effective feeding time.

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. This effect is very obvious intertidally on rocky shores, where the balance between macroalgae and grazing limpets changes completely with the degree of wave action, and limpets exclude fucoids from exposed areas where they can otherwise survive successfully (Jones, 1948).

The end result is that there are very different CFT biotopes in different conditions of 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. There is no need to enter into great detail here - the differences are to be readily seen from a comparison of the exposed and sheltered biotopes itemised in Appendix 1. Exposed areas tend to be dominated by coelenterates (e.g. Alcyonium digitatum, Tubularia indivisa, Corynactis viridis) and massive sponges (e.g. Pachymatisma johnstoni, Cliona celata). In contrast very sheltered communities are often dominated by ascidians (Ascidia mentula, Ciona intestinalis) and more delicate sponges (Suberites carnosus, Polymastia boletiformis). These are only general trends - under certain conditions coelenterates can dominate in shelter, and ascidians in exposure. In exposed sites differences will be found between those receiving extreme wave exposure (jewel anemones, bryozoan turfs, massive sponges, soft corals) and very strong tidal currents (barnacles, and the hydroid Tubularia). Hiscock (1985) presents very comprehensive lists of circalittoral rock species found under different conditions of water movement, discriminating the effects of waves and currents.

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