Biodiversity & Conservation

The dominant and characterizing species in the biotope (Macoma balthica and Hediste diversicolor) are infaunal and display plasticity in their feeding methods (McLusky & Elliott, 1981; Nielsen et al., 1995). They are primarily deposit feeders but are able to switch to suspension feeding when conditions allow. For example, in the case of Hediste diversicolor, if phytoplankton concentrations are sufficiently high the species switches from predatory and surface deposit-feeding to suspension feeding. This behaviour is maintained as long as the phytoplankton concentration remains above a 'trigger level' of 1-3 µg chlorophyll a per litre (reviewed by Riisgard, 1994).

Obligate deposit feeders are also numerous in the biotope, e.g. Pygospio elegans, Aphelochaeta marioni, Arenicola marina and Hydrobia ulvae. Competition for resources is likely to occur between the deposit feeders. For example, densities of the amphipod, Corophium volutator, and the gastropod, Hydrobia ulvae, are strongly negatively correlated although the mechanism of the interaction is not well understood (Barnes & Hughes, 1992).

Furthermore, Corophium volutator and the infaunal annelid species in the biotope probably interfere strongly with each other. Adult worms probably reduce amphipod numbers by disturbing their burrows, while high densities of amphipods can prevent establishment of worms by consuming larvae and juveniles (Olafsson & Persson, 1986). Arenicola marina has been shown to have a strong negative effect on Corophium volutator due to reworking of sediment causing the amphipod to emigrate (Flach, 1992), and also on Pygospio elegans (Reise, 1985).

Suspension feeding bivalves , e.g. Cerastoderma edule, Mya arenaria and Abra tenuis, may occur where hydrodynamic conditions allow, i.e. in areas of stronger flow with coarser sediments.

Important epibenthic predators in the biotope include the shore crab, Carcinus maenas, and shrimps, e.g. Crangon crangon, which take infaunal populations of small bivalves, polychaetes and crustaceans (Elliot et al., 1998). Carcinus maenas has been shown to significantly reduce the numbers of Manayunkia aestuarina on mudflats (McClusky, 1989) and along with Crangon crangon may reduce the population of Corophium volutator in estuaries by more than 50% (Pihl, 1985).

Intertidal mudflats are important feeding areas for many fish species. The most significant predators are probably the flatfish, including sole, Solea solea, dab, Limanda limanda, flounder, Platichthys flesus, and plaice, Pleuronectes platessa, which feed on polychaetes and their tails (e.g. of Hediste diversicolor and Arenicola marina), bivalve young and siphons (e.g. of Macoma balthica) and crustaceans (see review by Elliot et al., 1998).

Mobile epibenthic predators are probably responsible for preventing the infauna reaching carrying capacity. For example, exclusion experiments by Reise (1985) on intertidal mudflats showed small fish and decapod crustaceans can significantly reduce numbers of Cerastoderma edule, Tubificoides benedii, Pygospio elegans, Aphelochaeta marioni, Eteone longa and Corophium volutator.

Carnivorous annelids, e.g. Eteone longa and Nephtys hombergii, operate at the trophic level below Carcinus maenas (Reise, 1985). They predate the smaller annelids and crustaceans in the biotope.

Macroalgae, such as Ulva lactuca and Ulva sp., probably only occur in the biotope where suitable hard substrata exist for attachment. However, Ulva lactuca may grow free floating in very sheltered conditions.

Seasonal changes are likely to occur in the abundance of fauna in the biotope due to seasonal recruitment processes and variations in recruitment success. For example, in the case of Macoma balthica, Bonsdorff et al. (1995) reported juvenile density in the Baltic Sea following settlement in late summer to be 300,000/m² decreasing to a stable adult density of 1,000/m², and Ratcliffe et al. (1981) reported adult densities in the Humber Estuary, UK, between 5,000/m² and 40,000/m² depending on time since a successful spat fall. Variation in abundance is also very pronounced in the polychaete Aphelochaeta marioni. For example, in the Wadden Sea, peak abundance occurred in January (71,200 individuals per m²) and minimum abundance occurred in July (22,500 individuals per m²) following maximum spawning activity between May and July (Farke, 1979). However, the spawning period varies according to environmental conditions and so peak abundances will not necessarily occur at the same time each year. For example, Gibbs (1971) reported Aphelochaeta marioni spawning in late autumn in Stonehouse Pool, Plymouth Sound.
Some species make seasonal migrations in response to environmental conditions. For example, Beukema & De Vlas (1979) reported that 30% of the Macoma balthica population migrated into the subtidal during winter apparently in response to low temperatures. Migration is achieved by burrowing (Bonsdorff, 1984; Guenther, 1991) and/or floating (Sörlin, 1988) .
Macroalgal cover typically varies through the year due to changes in temperature and light availability. The green macroalgae in the biotope are likely to proliferate in the spring and summer and die back in the autumn and winter in conjunction with decreasing light levels and temperature and increased disturbance by storm events. Production by microphytobenthos and microalgae is also likely to be higher in spring and summer, increasing food availability for deposit feeders and suspension feeders in the biotope.
The biotope is likely to be susceptible to increased wave action during storms, particularly in winter. Storms may result in changes in sediment composition and washing out of infauna, leaving the biotope available for recolonization (see review by Hall, 1994). The infauna may be damaged by wave action, displaced from their preferred habitat and/or cast ashore, resulting in mortality. For example, Tamaki (1987) studied the passive transport by waves and tidal currents of the adults of 5 polychaete species. One species exhibited a landward shift in its centre of population during winter when the wave effects were most profound, and reoccupied its summer position by active migration of adults.

• The substratum in the biotope is uniform sediment with little structural diversity provided by either physiographic features or the biota. Some 3-dimensional structure is provided by the burrows of infauna such as Arenicola marina. Most species living within the sediment are limited to the area above the anoxic layer, the depth of which will vary depending on sediment particle size and organic content. However, the presence of burrows allows a larger surface area of sediment to become oxygenated, and thus enhances the survival of a considerable variety of small species (Pearson & Rosenberg, 1978).
• Reworking of sediments by deposit feeders increases bioturbation and potentially causes a change in the substratum characteristics and the associated community (e.g. Rhoads & Young, 1970). For example, Widdows et al. (1998) reported that typical abundances of Macoma balthica increased sediment resuspension and/or erodability four fold and that there was a significant positive correlation between density of the species and sediment resuspension.
• Where present, the macroalgae provide some structural complexity in the habitat, providing cover and sites for attachment of epifauna and epiphytes.

• Primary production in the biotope comes from benthic microalgae and water column phytoplankton (Elliott et al., 1998).
• Production by benthic unicellular and filamentous algae in the littoral zone accounts for 0.2-1.3 g C/m² daily, depending on water clarity (Barnes & Hughes, 1992). Benthic microalgae are able to photosynthesize over a much wider range of light intensity than the planktonic species. There is little or no photoinhibition, adapting littoral species to the full sunlight experienced at low tide while still utilizing the very low light intensities at high tide (Barnes & Hughes, 1992).
• Where present, macroalgae also contribute to primary production in the biotope. They exude considerable amounts of dissolved organic carbon which are taken up readily by bacteria and possibly by some larger invertebrates.
• The majority of nutrients in the biotope are derived from allochthonous sources. Mudflats receive large inputs of nutrients, sediment and organic matter from the sea and land discharges of river water and sewage, resulting in high productivity despite low levels of primary production (Elliott et al., 1998).

The bivalves which characterize the biotope are capable of high recruitment and rapid recovery. For example, adult Macoma balthica spawn at least once a year and are highly fecund (Caddy, 1967). There is a planktotrophic larval phase which lasts up to 2 months (Fish & Fish, 1996) and so dispersal over long distances is potentially possible given a suitable hydrographic regime. Following settlement, development is rapid and sexual maturity is attained within 2 years (Gilbert, 1978; Harvey & Vincent, 1989). In addition to larval dispersal, dispersal of juveniles and adults occurs via burrowing (Bonsdorff, 1984; Guenther, 1991), floating (Sörlin, 1988) and probably via bedload transport (Emerson & Grant, 1991). It is expected therefore that recruitment can occur from both local and distant populations.
The infaunal polychaetes Hediste diversicolor, Arenicola marina and Aphelochaeta marioni have high fecundity and the eggs develop lecithotrophically within the sediment or at the sediment surface (Farke, 1979; Beukema & de Vlas, 1979). There is no pelagic larval phase and the juveniles disperse by burrowing. Recruitment must occur from local populations or by longer distance dispersal of postlarvae in water currents or during periods of bedload transport. For example, Davey & George (1986), found evidence that larvae of Hediste diversicolor were tidally dispersed within the Tamar Estuary over a distance of 3 km, as larvae were found on an intertidal mudflat which previously lacked a resident population of adults. Recruitment is therefore likely to be predictable if local populations exist but patchy and sporadic otherwise.
The deposit feeding gastropod Hydrobia ulvae appears to display plasticity in its developmental mechanism. Fish & Fish (1996) report planktotrophic development with a free-swimming larval phase lasting 3 weeks, while Pilkington (1971) stated that development occurred via a non-feeding benthic larvae which metamorphosed in just 3 days. It is possible that Hydrobia ulvae is able to change its developmental mechanism according to environmental conditions. If conditions are favourable, the eggs may hatch, develop directly and recruit locally. In more stressful conditions, it may benefit the individual to disperse its offspring more widely via a planktotrophic larva.
Recruitment of shallow burrowing infaunal species can depend on adult movement by bedload sediment transport and not just spat settlement and juvenile dispersal. Emerson & Grant (1991) investigated recruitment in Mya arenaria and found that bedload transport was positively correlated with clam transport. They concluded that clam transport at a high energy site accounted for large changes in clam density. Furthermore, clam transport was not restricted to storm events and the significance is not restricted to Mya arenaria recruitment. Many infauna, e.g. polychaetes, gastropods, nematodes and other bivalves, will be susceptible to movement of their substratum.

No information was found concerning time taken for the community to reach maturity. However, the characterizing species are highly fecund and quick to grow and mature and so the community would be expected to reach maturity within 5 years.