Economic importance

Scientific importance

Biodiversity and conservation importance

The ‘Sea pens and burrowing megafauna’ biotope complex is of considerable economic importance because of the fishery targeted on one of its principal constituent species, Nephrops norvegicus (Howard, 1989). Nephrops was almost unexploited prior to the 1950s, but since then has grown rapidly in importance to the UK fishing fleet, with Scottish landings in 1995 totalling 22476 tonnes, with a market value of £47.6 million (Marine Laboratory, Aberdeen). In 1995 the most important fishing areas around the UK were the western Irish Sea (about 8000 tonnes), the Fladen Grounds in the North Sea (7087 tonnes), the North and South Minches (3656 and 4678 tonnes respectively) and the Clyde (3989 tonnes), with important fisheries also in the Firth of Forth, Moray Firth and eastern Irish Sea. Bottom trawling is the main technique used, but inshore creel fishing is also locally important, particularly on the west coast of Scotland.

The squat lobster, Munida rugosa, also supports a small commercial fishery. Reported annual landings are currently only about 10 tonnes, mostly from the Clyde area (C.J. Chapman, personal communication), but this has the potential to increase if the market expands.

No other megafaunal burrower, or any sea pen species, has any economic importance in the UK. However, the activities of burrowing megafauna can impinge on human usage of the sea in other respects. It has become generally recognized over the past two decades that disturbance of the sea bed by animal activity can have major effects on the structure, movement and chemistry of marine sediments (McCall & Tevesz, 1982). These disturbance processes, collectively termed ‘bioturbation’ include the construction of burrows, the transport of material from deep sediment layers to the surface (or vice versa), and the sorting of sediment particles for feeding purposes. By circulating water through their burrows, benthic animals transport oxygen to deep, otherwise anoxic layers of sediment. In heavily-burrowed expanses of sea floor, burrow walls can collectively greatly increase the surface area of sediment in contact with the water column. Both factors can profoundly affect the types and rates of chemical reactions taking place at the sediment-water interface, in particular the recycling of nutrients such as nitrate and phosphate, and metals such as manganese. The burrowing megafauna are likely to be particularly important in these processes as a result of their large body size and the depth of their activity in the sediment (Nickell et al., 1995b).

The effects of burrowing animals on sediment-water chemistry take on a practical significance with respect to the fate of man-made pollutants discharged into the sea. Contaminants such as toxic metals, pesticides or radionuclides often become bound to sediment particles and accumulate in the sea bed. Bioturbation is one means by which sedimentary pollutants can be redistributed, and potentially returned to the human environment (Lee & Swartz, 1980), and for this reason the process has become a major area for research in recent years. In the UK, the possibility that benthic animals might significantly affect the distribution of radionuclides such as plutonium and caesium in the sediments of the Irish Sea was recognized in the early 1980s (Kershaw et al., 1983; Swift, 1993). This discovery greatly stimulated research into the ecology of species such as Maxmuelleria lankesteri, about which very little was previously known (Hughes et al., 1996b).

The increasing interest in bioturbation and its effects has led to a significant use of shallow coastal sediment areas as natural ‘laboratories’ where the ecology of burrowing animals can be studied under natural conditions. In this respect, several of the Scottish sea lochs (notably Loch Sween in mid-Argyll) have been especially important, as their organically-enriched sediments typically support a diverse burrowing megafauna in shallow, highly sheltered waters suitable for diving fieldwork (Nickell et al., 1995b). Data collected in sea lochs has been used to assess the relative importance of different species in the redistribution of radionuclide particles in the bottom sediments of the north-eastern Irish Sea (Hughes & Atkinson, 1997). Observations made in coastal environments can also help to interpret the behaviour of burrowing animals in the deep sea (eg. Ohta, 1984; Hughes et al., 1994). The ‘Sea pens and burrowing megafauna’ biotope complex is therefore of considerable scientific value as a model for studying processes of general importance in marine benthic ecology, and this value is likely to increase as further sites are investigated and additional species become available for study.

As outlined earlier, the three shallow-water sea pens all have wide distributions around the north-eastern Atlantic and Mediterranean, some extending further afield. Funiculina quadrangularis has a more restricted distribution in British coastal waters than the other two species and is considered to be of greater national conservation importance. Another nationally rare species found in the deep mud biotope is the large anemone Pachycerianthus multiplicatus, known in the British Isles only from some of the Scottish and Irish sea lochs/loughs (Howson et al., 1994).

The major megafaunal burrowers are also widely distributed. Species that currently appear to be rare or localized in distribution are in many cases probably merely overlooked or under-recorded. This situation is inevitable given the cryptic habits of these animals, the comparatively recent advent of techniques such as SCUBA and underwater video, and the small number of localities that have been surveyed in detail using these methods. The echiuran Maxmuelleria lankesteri remained in obscurity for decades after its formal description, known only from a handful of specimens, but recent work has shown it to be widespread and locally common where it occurs. The same will probably turn out to be true of other species about which little is currently known. It is therefore not possible at present to identify any burrowing megafauna likely to be confined to British waters, or truly rare on a national or international scale.

The soft-sediment biotopes with sea pens and burrowing megafauna known from the UK are, however, of conservation importance as the best-characterized and most intensively-studied examples of their kind. Comparable animal communities are known from elsewhere (see below), but work in British waters has progressed much further in identifying the principal species present (Atkinson & Nash, 1985; Atkinson, 1986), determining the structure of their burrows (eg. Atkinson & Chapman, 1984; Nash et al., 1984; Atkinson & Nash, 1990; Nickell et al., 1995a; Nickell & Atkinson, 1995; Rowden & Jones, 1995), and quantifying their effects on the sediment (Nickell et al., 1995b; Hughes et al., 1996a). This is particularly true of the communities found in the Scottish sea lochs, which provide favourable conditions for detailed study matched in few other places. Since the best-known examples of this biotope complex occur in shallow, semi-enclosed bodies of water, where they are potentially vulnerable to disturbance by a variety of natural and human-induced environmental changes (see Chapters V and VI), they could be used as indicators of the ‘health’ of the local coastal environment.

In terms of species number, the sediment-dwelling meio- and macrofauna of sedimentary habitats far exceed the larger, more conspicuous animals around which this biotope complex is defined. This component of the biological community is therefore also of potential conservation importance, although not usually discussed in this context. The interaction between the burrowing megafauna and these smaller organisms will be discussed in Chapter IV.

The conservation importance of this biotope complex will be assessed more fully in Chapter IX, after its ecological functioning and sensitivity to environmental change have been reviewed.

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