Although the effect of one or more environmental factors acting singly or in conjunction with others is important, the primary factor controlling the dynamics of the intertidal sand and mudflats and the subtidal mobile sand banks is the hydrophysical regime. The interactions of all physical factors will determine the composition and density of the infauna (Eleftheriou & McIntyre, 1976). Species in the biotope complexes are somewhat protected against the sedimentary instability and variability in temperature, salinity, exposure and predation by burrowing (Eagle, 1973).

Marine organisms have fundamental tolerances which dictate their large scale geographical distribution (Glemarec, 1973). On a regional scale, temperature tolerances will produce ‘biogeographical zones’ (e.g. Arctic, Boreal, Lusitanian assemblages) and salinity tolerances will dictate the extent of distributions within freshwater-affected environments such as estuaries (McLusky, 1989).

Unstable sediments support fewer organisms than stable ones and only those mobile species which can re-establish their position, e.g. the haustoriid amphipods and Eurydice pulchra, can survive in very unstable conditions (Allen & Moore, 1987). Some species of macrofauna, in particular the crustaceans, are adapted to living in sediments exposed to heavy wave action mainly through their ability to burrow rapidly (Brown, 1983). On NW European shores these are usually small amphipods such as Bathyporeia spp. and Pontocrates spp. which live in the water column but burrow into the sediment as the tide level falls. These organisms probably depend on the washouts in high-energy (mobile) sands for feeding and migration.

In general, decreasing exposure to wave action correlates with increases in abundance, species richness and biomass of polychaetes and a decrease in abundance of crustaceans (Dexter, 1990). Polychaetes are often limited to Nephtys spp., Scolelepis squamata and capitellids with molluscs either present in very low numbers or absent (Angus, 1979). Survival rates of organisms such as sedentary polychaetes, living in the sediment decrease when surface sediments are disturbed daily although it is possible that small ones are simply relocated (Brown, 1982). Motile species such as Scolelepis squamata, however, are adapted to life in unstable sediments and survive through rapid burrowing (McDermott, 1983).

Allen and Moore (1987) found correlations between community structure and the prevailing physical conditions including shore stability for both individual organisms and guilds. The relationships were more evident lower down the shore where other factors such as desiccation were less important. For example, Bathyporeia sarsi was found in stable and unstable conditions and Paraonis fulgens and Arenicola marina at moderately unstable sites. P.fulgens is able to anchor itself in the sediment and A. marina can burrow to depths of 0.2 m where wave action does not penetrate. Nephtys cirrosa was the only errant polychaete strongly associated with unstable sediments. Rasmussen (1973) and Wolff (1973) give the ecological preferences of many other intertidal and shallow organisms.

Species diversity as well as overall community structure, is influenced by the habitat stability and sediment type. Coarse sediments, which are unstable and difficult to burrow into, are dominated by epifauna, while fine sediments are increasingly dominated by infauna. Many species are found in or on a range of sediment types, but others have a more restricted distribution (Wolff, 1973). For example, each of the five species of the bivalve Nucula, which occurs in subtidal muddy-sands, prefers a particular grade of sediment (Wood, 1987). The greatest diversity of macro-infaunal species is generally associated with poorly-sorted sands because they are physically heterogeneous, and thus have a large number of ecological niches, are reasonably stable and contain a supply of deposited organic matter.

Sedimentary features influencing the distribution of feeding guilds, e.g. suspension and deposit feeding benthos (Sanders, 1958), where high silt-clay fractions (relating to greater amounts of food) partly explained the presence of the deposit-feeding benthos. Deposit feeders dominate over suspension feeders in areas with higher percentages of silt-clay. They feed on the bacterial and microphytobenthos film surrounding sand and mud particles and therefore tend to dominate mud flats and sheltered shores. A non-selective deposit feeder such as Arenicola marina is dependent on detritus as its main food supply while living micro-organisms providing nutrients such as vitamins. Nutrients from the faecal cast may be reabsorbed either directly or from the bacteria degrading the cast (termed ‘gardening strategy’).

The distribution of suspension feeders is greatly affected by sediment instability as muddy sediment and high turbidity clog the filtering organs. In addition, subtle changes in the relative proportions of sand/silt/clay will affect an organisms’ ability to maintain a burrow (Meadows & Tait, 1989). Suspension feeders such as Lanice conchilega tend to dominate more exposed shores and coarser sediments where food supply may be limited but constant and with their abundance determined by the supply of particulate organic material and plankton in the water (Brown, 1983; McLachlan, 1983; Peterson, 1991).

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