The biotopes within this complex are associated with physically-sheltered conditions of low wave exposure and weak tidal currents. They are therefore not generally subjected to major physical disturbance by storms even where they occur in relatively shallow water (eg. in many sea lochs). Examples of these biotopes occurring off more open coasts, such as in the North or Irish Seas, will generally be in water deep enough to be unaffected by storm events.

In shallow sea loch environments, sedimentary biotopes will typically experience seasonal temperature changes over a range of about 10^oC (from ~ 5 - 15^oC). In unusually cold winters or warm summers temperatures outside this range might be experienced, but it is not known whether this will have any effect on the biological community. Megafauna which burrow deeply in the sediment will probably be buffered to some extent from temperature changes in the overlying water column. The example of Echiurus echiurus in the German Bight suggests that annual variation in temperature can alter community composition by its effects on recruitment, but there is too little information available on other species to say whether this is a widespread phenomenon.

Warm summer temperatures may lead to stratification of the water column and to conditions of hypoxia - a reduction in dissolved oxygen content - in the near-bottom water, This is especially likely to occur in semi-enclosed water bodies such as sea lochs. Hypoxia will be exacerbated by high levels of organic matter in the sediment, and its potential effects on biological communities will be discussed later in association with organic enrichment.

Large increases in the organic content of the bottom sediments and associated phenomena such as oxygen depletion can have a significant effect on benthic animals of all types, including megafaunal burrowers. In the present context, organic enrichment is most likely to occur as a consequence of human activities such as sewage sludge disposal or cage aquaculture, and its effects will therefore be reviewed in the following chapter.

Well-known examples of benthic communities substantially affected by sudden population increases in predator or grazer species include the crown-of-thorns starfish (Acanthaster planci) ‘outbreaks’ on Indo-Pacific coral reefs (Moran, 1986), and the periodic devastation of northern hemisphere kelp beds by sea urchins (Hagen, 1995). So far, however, there have been no recorded examples of such events in the biotopes discussed here.

Like all organisms, the species making up the biological communities discussed here are subject to a variety of pathogenic (disease-causing) and parasitic infections. Thalassinidean mud-shrimps are frequently parasitized by specialized isopod crustaceans known as bopyrids. The parasite lives in the gill chamber of the mud-shrimp and can inhibit the reproductive development of its host (Tucker, 1930). In the southern North Sea, Rowden & Jones (1994) found that up to 11% of Callianassa subterranea individuals were infected with a bopyrid parasite. Presumably the reproductive output of mud-shrimp populations will be reduced to some extent by these parasites but it is not known whether this has any wider consequences.

To date, the most significant disease process recorded in a species from this biotope complex is the recent identification of a syndinean dinoflagellate pathogen, Hematodinium sp., in Nephrops norvegicus from the west of Scotland (Field et al., 1992; Appleton & Vickerman, 1998). The Syndinea is a group of exclusively parasitic dinoflagellates, and members of the genus Hematodinium have emerged as serious pathogens of commercially-important crustaceans in several areas of the world in the last decade (Shields, 1994). In Nephrops, the parasite occurs in the blood and connective tissue spaces and appears to cause death by blocking the delivery of oxygen to the host’s tissues (Taylor et al., 1996). Hematodinium is most prevalent in Nephrops during the spring and early summer when infected animals have an abnormal bright orange body colouration and milky white ventral abdomen caused by a dense concentration of parasite cells in the blood. Heavily-infected animals become moribund, spend more time out of their burrows than healthy animals and are probably less able to evade capture by predators or fishing gear. Heavy infestation with the parasite is fatal to the host.

In the Clyde Sea, peak occurrence of the disease occurred in 1991 and 1992, when up to 70% of trawled Nephrops were infected (UMBS Millport, 1996). The incidence then declined, with 10 - 20% occurrence at Clyde Sea sites in 1996 and 1997. Infestation rates in 1998 appear to have increased again (R.J.A. Atkinson, personal communication). The infestation is now also well-established in the Irish Sea and appears to be increasing in the North Sea. The ecological consequences of Hematodinium infestation and host mortality in Nephrops populations are unknown, but there are potential economic implications, since the disease adversely affects meat quality. Fortunately, infected meat is non-toxic, and fishing mortality largely masks the effects of disease mortality. So far, the Nephrops fishery has not suffered any serious decline. This parasitic infestation is probably not a new phenomenon, but was overlooked until the mid-1980s. Prevalence of the infection is probably cyclical, with a large peak in the early 1990s which prompted the recent increase in research. An intensive research programme is currently in progress to clarify the life cycle of the parasite, determine the mode of transmission and assess the consequences of the problem for the Nephrops fishery.

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