Biodiversity & Conservation

The ecology of this biotope is assumed to be similar to that of most muddy habitats. Sheltered areas accumulate fine muds and silts, which in turn accumulate organic material in the form of detritus from the surrounding habits (e.g. from settling phytoplankton, plants and macroalgae) and from the surrounding catchment area via riverine or pluvial water flow (Elliot et al., 1998).

Fine muds and silts adsorb organic material and provide a large surface area for microbial colonization by heterotrophic microbes that degrade organic matter. However, the fine silts and muds result in low permeability, trapping detritus, and the large microbial population leads to high oxygen uptake, deoxygenation of the sediment and low degradation rates. Therefore, fine muds accumulate organic matter (Elliot et al., 1998).

Deoxygenation results in anaerobic degradation (releasing hydrogen sulphide, methane and ammonia) in the deeper sediment but release dissolved organic matter and nutrients for consumption by the aerobic microbes living at the surface (Elliot et al., 1998).

The large surface area of the particulates support microflora (microphytobenthos), such as diatoms and euglenoids, and this biotope may be covered by a diatom film (Connor et al., 1997a; Elliot et al., 1998).

The mud surface may be covered by ephemeral green algae such as Derbesia sp. (JNCC, 1999).

The majority of the macrofauna are deposit feeders that consume meiofauna, organic detritus, bacteria and microphytobenthos (e.g. diatoms) in the sediment. Arenicola marina and Leptosynapta sp. are 'funnel feeding' surface deposit feeders, ingesting sediment from the base of a funnel of sediment from within a U-shaped burrow (see Arenicola marina review; Hyman, 1955; Wells, 1945; Lawrence, 1987; Massin, 1982a, 1982b; Zebe & Schiedek, 1996).

Labidoplax media lives in the first few cm of sediment, ingesting sediment particles picked up by its tentacles (Gotto & Gotto, 1972). Where present, terebellids feed on surface deposits.

Arenicola marina and holothurians are known to take up dissolved organic matter (DOM).

Mobile species are opportunistic scavengers and predators and include starfish (e.g. Asterias rubens), crabs and hermit crabs (e.g. Carcinus maenas and Pagurus bernhardus), flatfish and gobies (e.g. Pomatoschistus minutus). Flatfish are important predators on intertidal mudflats and are presumably of similar importance in this biotope taking individual Labidoplax sp., Leptosynapta sp. and polychaetes.

Flatfish are known to 'nip' the tails of Arenicola marina. This is a shallow (0-5m) biotope and some predation from wading birds and wildfowl probably occurs, although no information was found.

Barnes (1980) provides a generalised food web for lagoonal habitats, which may be similar to the food web expected within this biotope.

Microphytobenthos and algal production may increase in spring, resulting in the formation of mats of ephemeral algae, and be reduced in winter. High summer temperatures may increase the microbial activity resulting in deoxygenation (hypoxia or anoxia), or alternatively result in thermoclines in shallow bays and resultant hypoxia of in the near bottom water (Hayward, 1994; Elliot et al., 1998). Flatfish and crabs often migrate to deeper water in the winter months, and therefore, predation pressure may be reduced in this biotope. Mud habitats of sheltered areas are relatively stable habitats, however especially cold winter or hot summers could adversely affect the macrofauna (see sensitivity). In addition, extreme freshwater runoff resulting from heavy rains and storms may result in low salinity conditions. No information on long term change was found.

Sheltered sediments, (such as found in this biotope) are characterized by fine grain size, low porosity, generally low permeability (and hence high water content), high sediment stability (due to cohesion), a low oxygen content and highly reducing conditions (Elliot et al., 1998). The mud surface is oxygenated. However, in fine muds, the anoxic reducing layer is likely to be very close to the surface, often less than 1cm (Elliot et al., 1998). Bioturbation by burrowing species, especially Arenicola marina and to a lesser extent Leptosynapta sp. results in mobilisation of the sediment and nutrients from deeper sediment to the surface, making nutrients available to surface dwelling organisms. In addition, continued irrigation of their burrows by Arenicola marina and Leptosynata sp. transports oxygenated water into the sediment, resulting in oxygenated micro-environments in the vicinity of their burrows.

In high enough abundances bioturbation by Arenicola marina (and to a lesser extent Leptosynapta sp.) modifies the sediment surface into mounds or casts and funnels. The resultant increase in bed roughness may result in increased susceptibility to erosion since raised features provide sites where areas of turbulent flow can be initiated. However, the effects of mucus binding in faecal pellet deposits increase the cohesiveness of the sediment, reducing its susceptibility to erosion (see Hall, 1994). For example, Leptosynapta tenuis was reported to increase the stability of the upper 3cm of sediment, while decreasing the stability of sediment below 3-10cm (Massin, 1982b). In addition, pits may capture fine detritus, resulting in increase microbial production within the pit. Surface sediment cohesion is also increased by the mucus binding of diatoms, bacteria and meiofauna (Hall, 1994).
The habitat can be divided into the following niches:

• a mobile epifauna of scavengers and opportunistic predators;
• a sediment surface of ephemeral green algae in the summer months;
• a sediment surface flora of microalgae such as diatoms and euglenoids, together with aerobic microbes;
• an aerobic upper layer of sediment (depth depending on local conditions but possibly <1cm) supporting shallow burrowing species such as Labidoplax media or Labidoplax buskii;
• a reducing layer and a deeper anoxic layer supporting chemoautotrophic bacteria, and
• burrowing polychaetes (e.g. terebellids and Arenicola marina) and burrowing synaptids (e.g. Leptosynapta sp.) that can irrigate their burrows.

Primary productivity is provided by microphytobenthos, epifloral algae and settling phytoplankton. However, the majority of productivity will be secondary, from the consumption of detritus and other organic material. Microbial degradation of detritus makes primary production readily available for animal consumption (McLusky, 1989; Elliot et al., 1998). No information concerning overall productivity in this biotope was found.

Sessile or sedentary species in the biotope recruit from planktonic propagules (larvae and spores) of the characteristic species.

Arenicola marina has a high fecundity and spawns synchronously within a given area, although the spawning period varies between areas. Spawning usually coincides with spring tides and fair weather (high pressure, low rainfall and wind speed) (see Arenicola marina review). Beukema & de Vlas, (1979) suggested an average annual mortality or 22%, an annual recruitment of 20% and reported that the abundance of the population had been stable for the previous 10 years. However, Newell (1948) reported 40% mortality of adults after spawning in Whitstable. McLusky et al. (1983) examined the effects of bait digging on blow lug populations in the Forth estuary. Recovery occurred within a few months by recolonization from surrounding sediment (Fowler, 1999). Beukema (1995) noted that the lugworm stock recovered slowly from mechanical dredging reaching its original level in at least three years.

Overall, therefore recovery is generally regarded as rapid, and occur by recolonization by adults or colonization by juveniles from adjacent populations. However, Fowler (1999) pointed out that recovery may take longer on a small pocket, isolated, beach with limited possibility of recolonization from surrounding areas. Therefore, if adjacent populations are available recovery will be rapid. However, where the affected population is isolated or severely reduced (e.g. by long-term mechanical dredging), then recovery may be extended. taking up to 5 years.

Little information concerning reproduction and recruitment in synaptid holothurians was found. Recruitment is echinoderms shows both temporal and spatial variability and is often sporadic and unpredictable. The role of pelagic predators and post settlement mortality is poorly understood (see Ebert, 1983 and Smiley et al., 1991 for reviews).

Labidoplax sp. and Leptosynapta sp. are hermaphrodites. For example, in Labidoplax buskii, male and female gametes are spawned on different days, presumably with some chemical mediated synchrony (Nyholm, 1951). Eggs are shed and sink to the sediment surface. Development is direct and lecithotrophic, forming a free swimming gastrula after about 2 days. Larvae are pelagic for 10-12 days, forming a pentactula larvae, returning to the sediment surface after 11-14 days in the laboratory (Nyholm, 1951). Nyholm (1951) stated that juveniles grew rapidly in their first 2 months of benthic life, reaching sexual maturity after 1 yr. In the Gullmar Fjord, the breeding season was between October to January, within a maximum in November and December (Nyholm, 1951). Nyholm (1951) considered the dispersal to be 'good'. Gotto & Gotto (1972) reported an increase in gonadial development in spring and summer in Labidoplax media from Strangford Lough, Ireland but observed only eggs and suggested that it was a protandrous hermaphrodite. Leptosynapta inhaerens was reported to breed between Aug and September in Norway (Smiley et al., 1991).

Although, the pelagic nature of propagules provide a potential for good dispersal, this biotope is found in particularly sheltered environments with limited water flow. Therefore, populations may be self recruiting and should a population be severely reduced it may take some time for recolonization to occur from other populations.

No information concerning community development was found. However, it is likely to depend on the species present, the hydrographic regime and recruitment and is likely to be highly variable between locations (see above).

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