Biodiversity & Conservation

The infauna established under the prevailing environmental conditions has the capacity to modify the sedimentary regime through activities that primarily effect the stability of and sedimentation within the habitat, for example by tube building, bioturbation, feeding behaviour and production of faeces and pseudofaeces. Such processes modify the habitat and increase the number of niches available for colonization (Elliott et al., 1998). Increased species diversity and abundance are known to occur around biogenic structures, such as the tubes of polychaetes, in otherwise relatively homogenous sedimentary habitats (Woodin, 1978). Studies of Lanice conchilega aggregations in the Wadden Sea ( Zühlke et al.,1998; Dittmann, 1999; Zühlke, 2001) showed that tubes built by Lanice conchilega had 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. There is considerable interaction between the species in this biotope; some are listed below.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. The abundance of predatory polychaetes (Eteone longa, Nephtys hombergii, Hediste diversicolor) was higher (Dittmann, 1999).

In sand, the primitive sea slug Acteon 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).

The predatory Nephytidae found within the biotope exert a negative effect on prey species. Beukema (1987) observed in long-term data from tidal flats in the westernmost part of the Wadden Sea, that Nephtys hombergii reduced the abundance and biomass of polychaetes Scoloplos armiger and Heteromastus filiformis. Schubert & Reise (1986) also reported similar evidence and concluded Nephtys hombergii to be an important intermediate predator.

The edible cockle, Cerastoderma edule, is the dominant bivalve within the biotope. Cerastoderma edule disturbs the upper sediment layer due to its crawling and regular "shaking" behaviour. Flach (1996) studied the effects of cockle behaviour on the recruitment of other benthic species, and found that the presence of Cerastoderma edule (even at a low density of 125-250 per m²) significantly reduced the densities of other bivalve species Macoma balthica, Mya arenaria, Angulus tenuis and Ensis spp., in addition to the worm species Pygospio elegans, Lanice conchilega, Etone longa, Anaitides spp., Nephtys hombergii, Heteromastus filiformis, Scoloplos armiger, Tharyx marioni and of the amphipods Corophium volutator and Corophium arenarium.

The occurrence of Lanice conchilega on tidal flats can be subject to high seasonal variations (Dittmann, 1999). Seasonal storms can cause displacement of the polychaete (Ropert & Dauvin, 2000) and in the intertidal the polychaete is known to be susceptible to severe winters (Strasser & Pielouth 2001). Populations of the cockle Cerastoderma edule are also periodically decimated by severe winter weather, and a high winter mortality is often followed by an exceptionally heavy spring spatfall (Hayward, 1994).

• 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 possibly ephemeral green algae in the summer months. The aerobic upper layer of sediment supports shallow burrowing species such as amphipods (Ampelisca spp., Bathyporeia spp. & Gammarus spp.) and small Crustacea, whilst the reducing layer and deeper anoxic layer support chemoautotrophic bacteria, burrowing polychaetes (e.g.Nephtys cirrosa, Nephtys hombergii, Arenicola marina and Magelona mirabilis), and burrowing bivalves (e.g. Cerastoderma edule).
• 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 hydrodynamics (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.

Biological production within intertidal sandbanks is highly variable being reliant on the quantity of nutrients being delivered or internally generated (Elliott et al., 1998). Some primary production comes from benthic microalgae and water column phytoplankton. The microphytobenthos in the interstices of the sand grains consist 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.

Characterizing macrofauna of the biotope are iteroparous, meaning that they breed several times per lifetime. Whilst some of the infauna have a benthic and brooding mode of reproduction (e.g. amphipods and oligochaetes), most are broadcast spawners (Rasmussen, 1973). For instance, polychaete worms including Lanice conchilega, Nephtys spp. and spionid worms release their eggs and sperm into the water where, after fertilization and a relatively prolonged planktonic phase of development, metamorphose and commence a benthic habit. Recruitment of Nephtys species seems related to environmental conditions in central parts of the species range, marginal populations exhibit occasional reproductive failures, e.g. Nephtys cirrosa, which is a temperate species and reaches the northern limit of its range in the north of the British Isles. Populations of Nephtys cirrosa on the east and west coasts of northern Britain exhibit different reproductive patterns. In south-west Scotland gravid adults breed every year in early autumn, whilst those on the east coast experience periods (e.g. over three years) of reproductive failure (Olive & Morgan, 1991). 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). For instance, adults of Cerastoderma edule spawn in a short peak period over summer with remaining adults spawning over a protracted period, resulting in a short (ca. 3 month) period of peak settlement followed by generally declining numbers of recruits (Hancock, 1967; Seed & Brown, 1977). For further information refer to full MarLIN species reviews.

The time required for the community to reach maturity will be in part determined by the proximity of other source populations and the season during which a disturbance occurs. Recolonization by some groups is likely to be more rapid than others. For instance, diatoms may be transported by resuspension in the water column and by lateral sediment transport. The rapid colonization (within days) by diatoms establishes food resources for other species, usually nematodes, that subsequently colonize. Dittmann et al. (1999) observed that the number of nematode species returned to pre-impact levels within seven days following a month long disturbance. Polychaetes tend to rapid colonizers, and species recorded by Dittmann et al. (1999) within two weeks included the polychaetes Pygospio elegans, Polydora sp., Nephtys hombergii, Capitella capitata, Heteromastus filiformis, Eteone longa, Hediste diversicolor (as Nereis diversicolor) and Scoloplos armiger, and the molluscs Macoma balthica and Mytilus edulis. Next to polychaetes, amphipods e.g. Urothoe poseidonis, are also rapid colonizers owing to their mobility. However, species that did not recolonize within the period of subsequent monitoring (14 months) included Arenicola marina, Lanice conchilega and its commensal Malmgreniella lunulata. Although it is likely that these species would recolonize suitable substrata, settlement of Lanice conchilega, for instance, has been reported to be more successful in areas with existent adults than areas without (see full MarLIN review; Heuers & Jaklin, 1999). Strasser & Pielouth (2001) reported that establishment of a mature population took three years in the absence of an established population. Thus the time taken for the community to reach maturity is likely to be in the order of several years.

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