Dynamics, threats and management of biogenic reefs
UNDER CONSTRUCTION
Contents
- 1 PROCESSES AND MECHANISMS DRIVING NATURAL DYNAMICS & ECOSYSTEM DEVELOPMENT
- 2 VULNERABILITY & THREATS
- 3 SEE ALSO
- 4 REFERENCES
PROCESSES AND MECHANISMS DRIVING NATURAL DYNAMICS & ECOSYSTEM DEVELOPMENT
In this section, the processes and mechanisms driving natural dynamics and ecosystem development of biogenic reefs are discussed for each group in turn: Sabellaria spinulosa, Sabellaria alveolata, Mytilus spp. and Modiolus modiolus.
Sabellaria spinulosa
Environmental Requirements
S.spinulosa is thought to require stable foundations on which to settle and establish a tube (Jackson, 1977[1]; Wood, 1999[2]; Chisholm and Kelley, 2001[3]) and is thus likely to favour substrata which include bedrock; boulders, cobbles, mixed substrata; and mixed sediment (Connor et al., 1997[4]). Although it is assumed that a firm substratum is required for colony establishment, it has been suggested that a reef can increase in extent without the need for hard substratum (Holt et al., 1997[5]). Many studies have reported extensive colonies in predominantly sandy areas (Warren and Sheldon, 1967[6]; Schäfer, 1972[7]; Warren, 1973[8]; Limpenny et al., 2010[9]). Recent observations from The Wash, England show that S. spinulosa had ‘seeded’ on shell fragments predominantly from blue or horse mussels (Ian Reach, Natural England, pers. comm.).
As S. spinulosa is a sedentary species, it relies on wave and current action to supply food and wash away waste products (Kirtley, 1992[10]). Strong water movement is required for food provisions, but is perhaps more important to raise sediment into suspension for tube building (Jones, 1999[11]). As a result, S. spinulosa colonies are typically located in areas of weak to moderately strong water flow (Jones et al., 2000[12]). It also appears to favour locations around the edges of sand banks or areas with sand waves (Foster‐Smith, 2001[13]). S. spinulosa typically occurs subtidally in depths of a few meters to up to 40 m depth (Caspers, 1950[14]; George and Warwick, 1985; Connor et al., 1997[4]; Jessop and Stoutt, 2006[15]), but can occur in depths up to 600 m (Hartmann-Schröder, 1971). S. spinulosa occasionally occurs in the lower intertidal zone (Jessop and Stoutt, 2006[15]).
Reproduction and Development
The fecundity and recruitment of S. spinulosa is known to be variable (e.g. Linke, 1951[16]; Wilson, 1971[17]; Michaelis, 1978[18]; George and Warwick, 1985[19]). The family Sabellariidae are broadcast spawners, reproducing sexually, resulting in larvae that drift passively in the plankton (Schäfer, 1972[7]; Eckelbarger, 1978[20]). The larvae can spend a few weeks to several months in the plankton (Wilson, 1929[21]) before seeking appropriate conditions for settlement (Wilson, 1968[22]; Eckelbarger, 1978[20]). If conditions are unsuitable, the larvae are able to delay metamorphosis for several weeks. Physical factors alone have limited influence on settlement (Wilson, 1968[22]) and settlement and metamorphosis is strongly influenced by the tube cement of other sabellariids (Wilson, 1968[22]; 1970[23]; Eckelbarger, 1978[20]; Jensen, 1992[24]). This mechanism ensures settlement in a suitable habitat and promotes the development of large colonies.
Despite only a few studies investigating the rate at which S. spinulosa can extend their dwelling tubes (Hendrick, 2007[25]; Davies et al., 2009[26] being exceptions), it appears that sabellariid reefs develop quickly following successful settlement (Linke, 1951[16]; Vorberg, 2000[27]; Stewart et al., 2004[28]; Braithwaite et al., 2006[29]). Last et al. (2011)[30] observed that tube extension rates are highly variable and that they could grow up to 6 mm a day for several days when provided with an adequate sediment supply.
Little is known about the longevity of S. spinulosa colonies, but sabellariids are expected to survive for 1-2 years (Kirtley, 1966[31]; McCarthy, 2001[32]; McCarthy et al., 2003[33]), with some reports of longer life spans (Wilson, 1974[34]; George and Warwick, 1985[19]). It is likely that the age of an actual colony may greatly exceed the age of the oldest individuals. This is particularly likely as sabellariid larvae are stimulated to metamorphose by conspecific secretions, encouraging continuous succession of generations.
Sabellaria alveolata
Environmental Requirements
S. alveolata generally requires hard substrata on which to develop, but these must be in areas with a good supply of suspended coarse sediment for tube building. S. alveolata reefs are known to form on a range of substrata from pebble to bedrock (Cunningham et al., 1984[35]). Reefs therefore commonly form on bodies of rock or boulders surrounded by sand. Larsonneur (1994)[36] noted that settlement of S. alveolata was facilitated by the sand mason Lanice conchilega which can stabilize sand well enough to allow colonization by S. alveolata. Settlement occurs mainly on existing colonies or their dead remains (Figure 1).
Water movement of sufficient intensity is a prime requirement to suspend coarse sand particles, thus making them available for the building of worm tubes. Cunningham et al. (1984)[37] note that this may consist of waves or currents. In many British localities such as the south west of England, much of Wales and the Cumbrian coast, the former seem more important. In other areas, such as parts of the Severn Estuary, tidal suspension is probably very important. However, S. alveolata is generally absent in very exposed peninsulas such as the Lleyn, Pembrokeshire and the extreme south west of Cornwall, which probably relates to the effect of water movement on recruitment (Cunningham et al., 1984[37]).
Reproduction and Development
It is thought that the larvae of S. alveolata spend 6 weeks to 6 months in the plankton (Wilson, 1968[22]; Wilson, 1971[17]) in order to attain widespread dispersal. The most detailed work done on S. alveolata reproduction in the British Isles is that of Wilson in Cornwall (e.g. Wilson, 1971[17]). Wilson observed slight settlement in every month except July, but in 14 years of monitoring (1961 to 1975), Wilson (1976)[38] observed only three heavy settlements: in 1966, 1970 and 1975. All occurred from September to November or December. Subsequent studies have revealed that the intensity of settlement is extremely variable, both temporally and spatially (Gruet, 1982[39]; Cunningham et al., 1984[37]). Settlement occurs mainly on existing colonies or their dead remains; chemical stimulation seems to be involved, and this can come from S. spinulosa tubes as well as from S. alveolata (Wilson, 1971[17]; Gruet, 1982[40]; Cunningham et al., 1984[37]).
Mytilus spp.
Environmental Requirements
The widespread distribution of the M. edulis is a reflection of its tolerance of a wide range of environmental variables. Natural reefs typically occur on firm, mixed sediments in relatively wave sheltered estuaries and bays characterized by strong currents (Holt et al., 1998). In more exposed areas, larger colonies are only able to develop on hard and stable substrata such as rock or large boulders (Seed, 1969). Conversely, in sheltered environments large beds may develop on more sandy substrates (Roberts and McKenzie, 1983).
Mussels produce byssal threads which anchor them to the substratum and each other, enabling large beds to develop. Mussels can grow in all but the most exposed conditions where their byssus threads can provide anchorage against wave action and water flow. As M. edulis is a sessile filter feeder, it requires sufficient water to flow to bring food and wash away waste. Larger beds require higher flow in order to provide sufficient food supply to high numbers of individuals. It is generally considered that this water movement is best provided by tidal currents rather than wave action, though the latter may also contribute in some areas (Holt et al., 1998).
M. edulis is tolerant of a wide range of salinities, being found in locations ranging from estuarine to fully marine, but larger reefs typically occur within the lower third of the intertidal and in the mid to lower reaches of the estuary (Holt et al., 1998). M. edulis reefs do form subtidally and have been reported to occur at depths of 30 m (Ian Reach, Natural England, pers. comm.). The upper limits of M. edulis are thought to be set by temperature and desiccations stress (Seed and Suchanek, 1992) in addition to reduced feeding (Widdows and Shick, 1985). The lower limits are generally set by biological factors such as competition and predation with physical factors playing a secondary role (Holt et al., 1998).
Reproduction and Development
Modiolus modiolus
Environmental Requirements
Reproduction and Development
VULNERABILITY & THREATS
GENERAL SUMMARY
Sabellaria spinulosa
Sabellaria alveolata
Modiolus modiolus
Mytilus spp.
NATURAL AND ANTHROPOGENIC THREAT
Sabellaria spinulosa
Physical threats
Chemical threats
Biological threats
Sabellaria alveolata
Physical threats
Chemical threats
Biological threats
Mytilus spp.
Physical threats
Chemical threats
Biological threats
Modiolus modiolus
Physical threats
Chemical threats
Biological threats
KEY PROCESSES TO FOCUS ON FOR MAINTAINING ECOSYSTEMS INTEGRITY
CURRENT MANAGEMENT PRACTICES
Sabellaria spinulosa
Sabellaria alveolata
Mytilus spp.
Modiolus modiolus
SEE ALSO
REFERENCES
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- ↑ LARSONNEUR C. 1994. The Bay of Mont‐Saint‐Michel: A sedimentation model in a temperate macrotidal environment. Senckenbergiana maritima. 24, 3‐63.
- ↑ 37.0 37.1 37.2 37.3 Cite error: Invalid
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- ↑ WILSON D.P., 1976. Sabellaria Alveolata (L.) At Duckpool, North Cornwall, 1975. Journal of the Marine Biological Association of the United Kingdom. 56, 305-310.
- ↑ GRUET Y., 1982. Recherches sur l’écologie des récifs d’Hermelles édicés par l’Annélide Polychète Sabellaria alveolata (Linné), Université des Sciences et Techniques, Nantes, France. PhD.
- ↑ Cite error: Invalid
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