Depth

Light

Initially depth would appear to be the paramount environmental factor determining the distribution of CFT communities. However, this is illusory in that depth per se is probably of little consequence. The organisms appear to be unaffected by pressure (the direct result of depth) within the depth range involved. If brought to the surface they live perfectly well at surface pressures - though this is not true of very deep sea organisms. Within the circalittoral zone depth exerts its effect only via its influence on other environmental factors. Light availability and the amount and type of water movement both vary greatly with depth. Temperature and salinity vary less, but the short term and annual fluctuations are both damped with increasing depth. These variables are all considered separately. The overall effect of increasing depth tends to be a reduction in fluctuations of most environmental factors, and it is now appreciated that in the deep sea this results in high biodiversity. This trend is apparent in circalittoral rock biotopes, with very high biodiversity evident in the deep circalittoral Lophelia-reef biotopes (Jensen & Fredriksen, 1992), where 298 species were recorded from a small area. With increasing depth hard substrata become scarcer, and as they appear to support diverse communities, they should be considered important in the context of SACs. The existing SACs include only limited areas of deep water, and this is perhaps a matter which needs consideration. However relatively deep CFT biotopes (>50 m) occur within the recommended boundaries of several of the candidate/possible SACs, including the following - Papa Stour; St Kilda; Lochs Duich, Long and Alsh reefs; Isles of Scilly.

Light is the environmental factor which basically determines the depth distribution of the circalittoral - the decrease of light with depth defines the upper limit of the zone. In areas where enough incident light reaches the sea bed the rock substratum community tends to be dominated by large macroalgae (the kelps) creating the infralittoral zone. When the light levels decline there is a progressive shift to faunal dominated communities. The reduction of animals in shallower depths is mainly due to competition for space with algae, though a few animals have symbiotic algae in their tissues and thus require light. These include the anemone Anemonia viridis and the hydroid Aglaophenia pluma (Hiscock, 1985). Areas of the infralittoral dominated by animal biotopes occur as a result of steep slopes, heavy grazing, and sometimes extreme physical conditions, however they are very much the exception.

The depth of the infralittoral-circalittoral transition depends on the penetration of light, which is influenced by a number of factors (Drew,1983). The main factors influencing water column light attenuation are the concentration of dissolved organic pigments and suspended matter, and these are both more abundant in coastal than in oceanic waters. Approximately 50% of the surface light reaches a depth of 10 m in the clearest oceanic water, but only 0.1% in very turbid coastal waters (Jerlov, 1976; Drew, 1983). The critical depth for the growth of kelp plants is at about 1% of surface illumination, and for foliose red algae about 0.1% (Luning & Dring, 1979). Consequently in turbid coastal waters the lower limit of abundant algal growth, and hence the upper limit of the circalittoral zone, will be shallower. Thus the main infralittoral canopy alga, the kelp Laminaria hyperborea, reaches its lower limit at 8 m in the relatively turbid waters of Helgoland, whilst it extends to about 20 m in the clearer waters of the Isle of Man (Kain, 1971), and in very clear water will grow down to over 30 m (Drew, 1983). In St Kilda kelps grow down to 47 m (Alistair Davison, pers. comm.). Water clarity, and its response to suspended matter, must be a major consideration in relation to the management of SACs.

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