Dynamics, threats and management of biogenic reefs

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UNDER CONSTRUCTION

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.


FFigure 1: Images illustrating the various stages of development of S.alveolata reef at Bude, Cornwall (photos L. Firth).


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)[35] 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[35]).


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)[37] 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[38]; Cunningham et al., 1984[35]). 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[38]; Cunningham et al., 1984[35]).


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[39]). In more exposed areas, larger colonies are only able to develop on hard and stable substrata such as rock or large boulders (Seed, 1969[40]). Conversely, in sheltered environments large beds may develop on more sandy substrates (Roberts and McKenzie, 1983[41]).

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[39]).

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[39]). 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[42], 1992) in addition to reduced feeding (Widdows and Shick, 1985[43]). 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[39]).


Reproduction and Development

The M. edulis fecundity and recruitment success is highly variable, both temporally and spatially. It can reproduce in its first year and can spawn throughout the year, with a major spawning event usually occurring in the spring (Seed, 1969[40]). Larvae can survive in the plankton for 2‐4 weeks before metamorphosis, although this can be up to 6 months, depending on availability of food, suitable substrate and temperature (Holt et al., 1998[39]). Settlement can be either a one-stage or a two‐stage process. Some larvae can settle directly onto adult beds (McGrath et al., 1988[44]) or they can temporarily settle onto sublittoral filamentous substrata such as algae or hydroids before becoming detached, and eventually settling onto an adult bed (Bayne, 1964; Pulfrich, 1996[45]). It is thought that this may be a mechanism for reducing competition between very young and adult mussels, and/or to prevent filtration of the larvae by the adult mussels. McGrath et al. (1988)[44] reported very large densities of settling spat in Ireland, but more commonly modest recruitment between the shells of adult mussels provides sufficient supply to maintain persistent beds (Holt et al., 1998[39]). Conversely, heavy recruitment may not necessarily lead to the formation or maintenance of a dense bed or reef if predation or losses due to wave action are high.

M.edulis growth and production can be extremely high, particularly in sheltered or estuarine areas (Holt et al., 1998). It has been reported that M. edulis accounts for 20% of the total macrobenthic production in the Wadden Sea (Beukema , 1981[46]), whilst Dare (1976)[47] estimated the production by two year classes to be 2.5‐3 times their maximum standing crop, with few mussels surviving beyond their third year. It is thought that the majority of mussels do not survive beyond 3 years of age (Seed, 1976[48]), there are reports of individuals surviving beyond 15 years (Sukhotin et al., 2007[49]).


Modiolus modiolus

Environmental Requirements

Despite typically occurring on hard substrata, M. modiolus beds and reefs are capable of forming on a variety of sedimentary bottoms, ranging from muddy substrata in some sea lochs to quite coarse mixed sediments containing much stones and shell. Larvae can also settle on artificial substrates such as oil rigs and can form reefs on these structures. The byssus threads of adult M. modiolus provide a suitable substrate for attachment and protection from predators. Beds occurring infaunally can lack available byssus threads and thus limit the recruitment (Holt and Shalla, 1997[50]) and the development of larger beds.

M. modiolus has a very wide depth distribution, typically being found subtidally from a few meters of depth right down to depths of 280 m (Schweinitz and Lutz, 1976[51]). Intertidal populations have occasionally been reported (Davenport and Kjosvik, 1982[52]), but these are thought to be limited by temperature and desiccation stress associated with aerial exposure (Coleman, 1976[53]; Davenport and Kjosvik, 1982[52]). The densest populations that are known as reef are found between 5 and 50 m in British waters (Holt et al., 1998), whilst infaunal reefs have been found at over 80 m in the Bay of Fundy (Wildish et al., 2009[54]).


Reproduction and Development

M. modiolus is a long-lived species with individuals only reaching sexual maturity between 3 and 6 years of age. It is thought that this adaptation is in response to high predation on juvenile mussels, thereby channeling energetic resources towards growth in early life. As a result, M. modiolus exhibits rapid growth in the first few years of life, followed by much slower growth following sexual maturation (Anwar et al., 1990[55]). M. modiolus spawning is known to be variable, both temporally and spatially. In Strangford Lough, Northern Ireland, slight spawning is known to occur year-round, with no apparent peak (Seed and Brown, 1977[56]; Brown, 1984[57]). Conversely, in Scandinavia, a spawning peak occurs in June, followed by a period of gonad redevelopment. Spawning is temperature dependent and is reported to occur within a narrow temperature range (7-10 °C). It is thought that the relatively constant temperatures in Strangford Lough facilitate the year-round spawning (Brown, 1984[57]). M. modiolus in the Irish Sea off the SE coast of the Isle of Man has been observed to follow an annual cycle of gonad development with a peak occurring in spring/summer, with trickle spawning occurring all year round (Jasim and Brand, 1989[58]).


VULNERABILITY & THREATS

GENERAL SUMMARY

This section is divided up into (1) the vulnerability and (2) the threats (biological, chemical and physical) to each species in turn: Sabellaria spinulosa; Sabellaria alveolata; Mytilus spp. and Modiolus modiolus. In this section, we refer to the sensitivity, vulnerability and potential for recovery of the habitat to sea level rise and storm events. In the case of natural reefs, flooding is not applicable and is therefore not discussed here. Much of the information from this section was sourced from the Marine Life Information Network website (www.marlin.ac.uk). We have adopted the terminology used by MarLIN with definitions below. In the following sections, we have identified the factors that are most likely to be associated with sea level rise and storm events for each species. The ‘intolerance’, ‘sensitivity’ and ‘recoverability’ of each species are presented in table format.

Intolerance is the susceptibility of a habitat, community or species (i.e. the components of a biotope) to damage, or death, from an external factor. Intolerance must be assessed relative to change in a specific factor.

Recoverability is the ability of a habitat, community, or species (i.e. the components of a biotope) to return to a state close to that which existed before the activity or event caused change.

Sensitivity is dependent on the intolerance of a species or habitat to damage from an external factor and the time taken for its subsequent recovery. For example, a very sensitive species or habitat is one that is very adversely affected by an external factor arising from human activities or natural events (killed/destroyed, 'high' intolerance) and is expected to recover over a very long period of time, i.e. >10 or up to 25 years ('low'; recoverability). Intolerance and hence sensitivity must be assessed relative to change in a specific factor.


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

Theseus Official Deliverable 3.3-Natural habitats for coastal protection and relevant multi-stressor coastal risks. Report and European Scale overview.

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The main authors of this article are Firth, Louise, Davies, Andrew, Hawkins, Stephan, Airoldi, Laura and Colangelo, Marina Antonia
Please note that others may also have edited the contents of this article.