Biodiversity & Conservation

The hydrodynamic regime and sediment composition and interaction between the two are probably the most significant factors structuring the community rather than biological interactions (Tyler, 1977; Warwick & Uncles, 1980; Elliott et al., 1998).

Species in the biotope are predominantly suspension and deposit feeders, and probably little direct interaction occurs between them other than competition for space.

Some species feed on both suspended particulates and surface deposits (e.g. the bivalves Fabulina fabula and Abra alba and the tube building polychaete Lanice conchilega. Following laboratory experiments, Buhr (1976) concluded that Lanice conchilega was capable of completely replacing deposit feeding by suspension feeding. The absolute amounts of food retained from suspension feeding and the assimilation efficiencies calculated were in the range typical for obligatory suspension-feeding organisms.

Studies of Lanice conchilega aggregations in the Wadden Sea ( Zühlke et al.,1998; Dittmann, 1999; Zühlke, 2001) have shown that tubes built by Lanice conchilega can have significant effects on the distribution, density and diversity of other macrobenthic species and meiobenthic nematodes compared to sites with a lower density of Lanice conchilega or ambient sediment without biogenic structures. The polychaete Harmothoe lunulata occurs in aggregations of Lanice conchilega and is often found inside the polychaetes' tubes, possibly being a commensal associated to Lanice conchilega (Zühlke et al., 1998). Juvenile bivalves (Mya arenaria, Mytilus edulis, Macoma balthica) were more frequent in patches with Lanice conchilega and settled especially on the tentacle crown of the worm tubes. In particular, abundances of predatory polychaetes (Eteone longa, Nephtys hombergii, Hediste diversicolor) were higher (Dittmann, 1999). The increased species diversity and abundance recorded in patches of Lanice conchilega are also known to occur around the tubes of other species of polychaetes (Woodin, 1978).

In sand, the primitive sea slug Faction tornatilis preys upon tube building polychaetes. A series of choice experiments suggested that the preferred prey items were the polychaetes Owenia fusiformis and Lanice conchilega (Yonow, 1989).

Lanice conchilega constitutes an important prey item for curlew, Numenius arquata, bar-tailed godwit, Limosa lapponica and grey plover, Pluvialis squatarola. The amphipods e.g. Ampelisca and Atylus species are probably epistatic grazers, grazing benthic microalgae from sand grains.

Amphipods and the infaunal annelid species in the biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows and tubes, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986).

Spatial competition probably occurs between the infaunal suspension feeders and deposit feeders. Reworking of sediment by deposit feeders makes the substratum less stable, increases the suspended sediment and makes the environment less suitable for suspension feeders (Rhoads & Young, 1970). Tube building by amphipods and polychaetes stabilizes the sediment and arrests the shift towards a community consisting entirely of deposit feeders. In the coarse sediments in this biotope the suspension feeding species dominate.

Amphipods are preyed upon chiefly by nemertean worms (see McDermott, 1984) and demersal fish (Costa & Elliott, 1991).

The abundant infauna are preyed upon by carnivorous polychaetes, e.g. phyllodocids species of Anaitides and Eumida, scale worms (e.g. Harmothoe sp.) and Nephtys hombergii. Asterias rubens, predates the bivalves (Aberkali & Trueman, 1985; Elliott et al., 1998).

Crabs, particularly Liocarcinus depurator and Carcinus maenas, are scavengers and predators of molluscs and annelids (Thrush, 1986; Elliott et al., 1998).

The hermit crab Pagurus bernhardus, brittlestars (e.g. Ophiura albida) and nemerteans are probably scavengers on detritus and carrion.

Gobies (e.g. Pomatoschistus species) and flatfish frequent the biotope to feed upon polychaetes, small crustaceans such as amphipods, cumaceans, small crabs, such predators also nip the siphons of bivalves and tails of polychaetes.

• Temporal changes are likely to occur in the community due to seasonal recruitment processes. For instance, in the German Bight, peak abundance of Fabulina fabula (ca 2000 individuals/m²) occurred in September following the main period of spatfall and then decreased to a minimum in February (ca 500 individuals/m²), at which point settlement began to occur again (Salzwedel, 1979). Similarly, temporal evolution of Fabulina fabula in NW Spain showed well marked annual peaks in autumn (Lopez-Jamar et al., 1995). In the German Bight, spatfall for Magelona mirabilis was heaviest in August/September, and for Echinocardium cordatum spatfall was heaviest in August (Bosselmann, 1989).
• Temporal variations in species richness and abundance are likely to occur due to seasonal patterns of disturbance, such as storms, harsh winters and oxygen deficiencies (Bosselmann, 1989; Lopez-Jamar et al., 1995). The biotope may also be liable to severe substratum disturbance, such as one in 25 year or one in 50 year storms, which can turn over sediment and completely disrupt the community (Elliott et al., 1998).
• There may be a spring-neap and winter-summer cycle of erosion and deposition of sediment, altering the biotope extent and reflecting changes in hydrodynamic energy (Dyer, 1998).
• The water temperature in subtidal sandy habitats may vary over 5-10 °C through the year in British coastal waters depending on depth. The variation may have short term but significant effects on species diversity (Buchanan & Moore, 1986).
• There is a seasonal variation in planktonic production in surface waters. Increased production by phytoplankton in spring and summer enhanced by increasing temperature and irradiance is followed by phytoplankton sedimenting events which correlate with seasonal variations in the organic content of benthic sediments (Thouzeau et al., 1996). Such variations directly influence the food supply of the deposit feeders and suspension feeders in the biotope.

Tidal streams in particular are a predominant factor influencing the structure of infralittoral sands, causing constant change in the shape, size and position and owing to the mobile nature of the sandy substratum and scour macrophyte communities do not become established.

The habitat can be divided into several niches. The illuminated sediment surface supports a flora of microalgae such as diatoms and euglenoids, together with aerobic microbes and ephemeral green algae in the summer months. The aerobic upper layer of sediment supports shallow burrowing species such as amphipods and small crustacea, whilst the reducing layer and deeper anoxic layer support chemoautotrophic bacteria, burrowing polychaetes (e.g. Chaetopterus variopedatus and Arenicola marina) that can irrigate their burrows, and burrowing bivalves (e.g. Abra alba).

In fairly homogeneous soft sediments, biotic features play an important role in enhancing species diversity and distribution patterns (Bandeira, 1995; Everett, 1991; Sebens, 1991). Polychaete dwelling tubes, such as those constructed by Lanice conchilega, provide one of the main habitat structures in the intertidal and subtidal zones. The tubes modify benthic boundary layer hydrodynamincs (Eckman et al., 1981), can provide an attachment surface for filamentous algae (Schories & Reise, 1993) and serve as a refuge from predation (Woodin, 1978) (Zühlke et al., 1998). Other biota probably help to stabilize the substratum. For example, the microphytobenthos in the interstices of the sand grains produce mucilaginous secretions which stabilize fine substrata (Tait & Dipper, 1998). The presence of infaunal polychaetes affects the depth of the oxic sediment layer. Tubes of Lanice conchilega and Arenicola marina can penetrate several tens of centimetres into the sediment. Such burrows and tubes allow oxygenated water to penetrate into the sediment indicated by 'halos' of oxidized sediment along burrow and tube walls. The burrow of Arenicola marina is irrigated (and therefore aerated) by intermittent cycles of peristaltic contractions of the body from the tail to the head end. Lanice conchilega is not known to purposely irrigate its tube since food acquisition occurs through surface deposit and suspension feeding, and respiration takes place via gills positioned outside the tube when the animal is feeding. However, the sand mason periodically withdraws from the surface into its tube for a few seconds, therefore acting as a 'piston pump', exchanging interstitial sediment water with overlying water (Forster & Graf, 1995).

Production in the biotope is mostly secondary, dependant upon detritus and organic material. Some primary production comes from benthic microalgae and water column phytoplankton. The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow in the upper several millimetres of illuminated sediments, typically appearing only as a subtle brown or green shading (Elliott et al., 1998). The benthos is supported predominantly by pelagic production and by detrital materials emanating from the coastal fringe (Barnes & Hughes, 1992). According to Barnes & Hughes (1992) the amount of planktonic food reaching the benthos is related to:

• depth of water through which the material must travel;
• magnitude of pelagic production;
• proximity of additional sources of detritus;
• extent of water movement near the sea bed, bringing about the renewal of suspended supplies;

In the relatively shallow waters around the British Isles secondary production in the benthos is generally high, but shows seasonal variation (Wood, 1987). Generally, secondary production is highest during summer months, when temperatures rise and primary productivity is at its peak. Spring phytoplankton blooms are known to trigger, after a short delay, a corresponding increase in productivity in benthic communities (Faubel et al., 1983). Some of this production is in the form of reproductive products.

The dominant species in the biotope are polychaetes and bivalves which, tend to have relatively long-lived planktonic larvae. More detailed information concerning recruitment of important characterizing species is given below:

Lanice conchilega is a polychaete species with separate sexes. During its life of 1-2 years (Beukema et al., 1978), the species initially passes through two larval stages, of which the last one, when it is known as an aulophora larva, lives about 4-6 weeks in the plankton (Kessler, 1963). Kuhl (1972) reported that the larvae of Lanice conchilega are released between April and October. Experimental data and field studies from the Wadden Sea revealed that the existence of 'hard substrate', preferentially tubes of conspecific adults, was a requirement for initial settlement of Lanice conchilega larvae, although single juveniles were also observed to settle on eroded shells of cockles (Cerastoderma edule) and soft-shelled clams (Mya arenaria) (Heuers, 1998; Heuers et al., 1998). Presumably this was the case following the ice winter of 1995/96 which decimated populations of Lanice conchilega on intertidal sand flats of the Dutch Wadden Sea, as settlement of larvae occurred in the absence of adults in the intertidal and shallow sublittoral (Strasser & Pielouth, 2001). Near bottom water velocity and turbulence are thought to be essential factors determining the spatial and temporal distribution of Lanice conchilega. According to Harvey & Bourget (1995) current velocity over a dense Lanice conchilega tube 'lawn' has to reach a value that causes turbulence and facilitates larval settlement. Lower current speeds reduce turbulence and thus the probability of larval settlement is reduced. The current speed at which turbulence developed depended on the density of the tube 'lawn', tube diameter and surface roughness in combination with currents. Grimm (1999) modelled the spatial and temporal distribution of Lanice conchilega with the intention of exploring mechanisms responsible for the large differences in density of the species on a tidal flat of the Dutch Wadden Sea. Field data collected by Brandt et al. (1995) seemed to tentatively confirm the model assumption made by Grimm (1999) that 'the local density of Lanice conchilega is strongly influenced by the overall velocity of the near bottom flow: low density occurs in areas where low velocities prevail, whereas high densities occur where high velocities prevail'. Brandt et al. (1995) recorded a mean flow velocity of 10 cm/s in low density stands of the species, and 20 cm/s near populations of high density.
• Hayward (1994) summarized recruitment of Arenicola marina. The species breeds late in the year and has a protracted spawning period between September and November, consisting of two peaks, in late September and late November, with a five week period between during which no spawning occurs. Production of eggs starts in February and March, so that by July the entire population consists of sexually mature adults, usually equal in number. Spawning is synchronous, induced by falling seawater temperature at a threshold of 13 °C, or by an abrupt downward temperature shock (Farke & Berghuis, 19790. Males are the first to spawn into their burrows, from which it is ejected to form pools on the sand. Females draw sperm into their burrows as they ventilate their burrows and eggs are fertilized. Development and hatching occurs within the female burrow. After spawning males fast for 2 days while females fast for 3-4 weeks, presumably to avoid ingesting eggs and larvae (Farke & Berghuis, 1979). The larvae leave the burrow when they have grown three chaete-bearing body segments. Post larvae are capable of active migration by crawling, swimming in the water column and passive transport by currents e.g. Günther (1992) suggested that post-larvae of Arenicola marina were transported distances in the range of 1 km. The species may live between 5 and 10 years.
• Reproductive data concerning Magelona mirabilis is scarce (Fiege et al., 2000). It is generally agreed that Magelona mirabilis displays characteristics typical of an r-selected species, i.e. high reproductive rate, short life span and high dispersal potential (Krönke, 1990; Niermann et al., 1990), and typically occurs in the early successional stages of variable, unstable habitats (Bosselmann, 1989; Niermann et al., 1990). Magelona mirabilis seems to have a protracted reproductive period. Fiege et al. (2000) recorded males with sperm masses and females with eggs in Scotland in March and egg bearing females in France in May. Probert (1981) reported Magelona sp. larvae in Plymouth Sound between April and November with greatest abundances from July to October, and Bosselmann (1989) reported that Magelona sp. larvae were captured in plankton trawls on most dates of sampling in the German Bight but were most abundant in August and September. Bosselmann (1989) also noted large interannual variability in numbers of Magelona sp. larvae in the plankton.
• The bivalves in the biotope are gonochoristic broadcast spawners with pelagic larval dispersal. They therefore have the potential to recruit both locally and remotely. However, bivalve populations typically show considerable pluriannual variations in recruitment, suggesting that recruitment is patchy and/or post settlement processes are highly variable (e.g. Dauvin, 1985). Abra alba is generally considered to be an 'r-strategist'; capable of rapidly exploiting any new or disturbed substratum suitable for colonization through larval recruitment, secondary settlement of post metamorphosis juveniles or redistribution of adults following storms (Rees & Dare, 1993). Normally, there two distinct spawning periods, in July and September and according to the season of settlement, individuals differ in terms of growth and potential life span (Dauvin & Gentil, 1989). Autumn settled individuals from the Bay of Morlaix, France, initially showed no significant growth; they were not collected on a 1 mm mesh sieve until April, 5 to 7 months after settlement. Such individuals were expected to have a maximum life span of 21 months and could produce two spawnings. In contrast, veliger larvae that settled during the summer grew very rapidly and were collected on a 1 mm mesh sieve just one month after settlement. They lived about one year and spawned only once (Dauvin & Gentil, 1989).

The life history characteristics of the species, particularly the polychaetes, which characterize the biotope suggest that the community would probably reach maturity within 3 years. For instance, settlement of Lanice conchilega has been reported to be more successful in areas with existent adults than areas without (Heuers & Jaklin, 1999), however, Strasser & Pielouth (2001) reported settlement in location without adults, but that establishment of a mature population took three years. Adults of Arenicola marina reach sexual maturity by their second year (Newell, 1948; Wilde & Berghuis, 1979) but may mature by the end of their first year in favourable conditions depending on temperature, body size, and hence food availability (Wilde & Berghuis, 1979). The life history characteristics of Abra alba and its widespread distribution contribute to its powers of recoverability. Diaz & Castaneda et al., (1989) experimentally investigated recolonization sequences of benthic areas over a period of one year following defaunation of the sediment. Recovery of the Abra alba population was rapid, recruitment occurred from surrounding populations via the plankton. The abundance, total biomass and diversity of the community all increased until a maximum was reached after 20 to 24 weeks, according to the season. The community within the experimental containers matched that of the surrounding areas qualitatively but quantitatively within 4 to 8 months depending on the seasonal availability of recruits, food supply and faunal interactions. The experimental data suggests that Abra alba would colonize available sediments within the year following environmental perturbation. Summer settled recruits may grow very rapidly and spawn in the autumn, whilst autumn recruits experience delayed growth and may not reach maturity until the following spring/summer.

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