Transparency and water clarity are affected by both dissolved material and suspended particles in the water, and are important because they influence the penetration of light. This has been discussed above. Suspended particles may be of natural origin, but can also arise from anthropogenic impacts such as dredging.

Suspended material in the water can settle out of the water column, and this can affect both the settlement and the survival of CFT species. It is normally only a problem in shelter, and the slope of the rock is also important: little sediment will settle on verticals or overhangs (Hiscock & Hoare, 1975). Silt influenced CFT communities are generally dominated by ascidians and sponges (Collins & Mallinson, 1987; Emblow et al., 1994), though ascidians can be sensitive to siltation, particularly in the settlement stage (Lundälv, pers. comm). Overall the effect of increased siltation on circalittoral rock biotopes is to lead to a decrease in diversity, and any factor increasing siltation would be a serious management concern.

Material in suspension can affect the efficiency of filter feeding (Sherk, 1971; Morton, 1977), and most of the CFT species are filter feeders (see section III.B). Effects can include abrasion and clogging of gills, impaired respiration, clogging of filter mechanisms, and reduced feeding and pumping rates. Decreased growth rates in species experimentally exposed to suspended sediment have been reported (Lawrence, 1993), though other reports claim that effects are generally small (Mackin, 1961; Saila et al., 1972). Obviously results will differ depending on the species studied, sediment loadings, and duration of exposure. Relevant studies have been made on the typical CFT species Alcyonium digitatum, using sediment loads similar to those induced by dredging activity, and lasting 2 months (Hill et al., 1997). No adverse effects were seen, though the colonies did occasionally slough off layers of sediment-coated mucus. However, not all CFT species may be as tolerant.

In exposed situations suspended material can cause scour, but this is normally a result of the temporary resuspension of relatively coarse bottom material rather than of fine material in long-term suspension. See 'scour' as an environmental factor.

Scour is a factor in more exposed areas where the rock substratum is in proximity to fine sediment. Typically such situations are found where boulders lie on a sandy bottom, or in the regions where the bedrock merges with the level sea bed. The existence of impoverished communities of tolerant species under such conditions has long been recognised (Forster, 1961; Knight-Jones & Nelson-Smith, 1977; Hiscock, 1979a; Hoare and Peattie (1979). A common feature in these communities is the presence of the hornwrack, Flustra foliacea. In the MNCR classification a variety of moderately exposed 'sand-influenced' biotopes are described as 'bryozoan/hydroid turfs'. In all of these biotopes the bryozoan Flustra foliacea is a prominent element, together with the calcareous tube worm Pomatoceros triqueter, and hydroids such as Tubularia indivisa and Nemertesia spp. At the rock-sand interface the community is often dominated by the anemone Urticina felina.

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