Exposure to water movement
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
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
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.
Detrimental effects include:
- 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.