Biological production of these areas is highly variable and relies on the quantity of nutrients being delivered or internally generated. Estuarine mudflats receive primary production from benthic microalgae (microphytobenthos: diatoms, flagellates and euglenoides) and water-column phytoplankton but this production may be light limited in these turbid environments. The mudflats receive a large input of nutrients, sediment and organic matter from the sea and land discharges of river water and sewage etc. and thus have a large productivity albeit from the low diversity created by salinity stress. More exposed sandflats are less productive as they are both harsher environments and with lower levels of organic matter. The intertidal mudflats in estuaries often have a higher production than subtidal areas as shown in the Forth Estuary (McLusky et al, 1992; Elliott & Taylor 1989b).

Intertidal sandflats only supports microphytobenthos in the interstices of the sandgrains. Mucilaginous secretions produced by these algae may stabilise fine substrata (Tait & Dipper, 1998). The microphytobenthos consists of unicellular eukaryotic algae and cyanobacteria that grow within the upper several millimetres of illuminated sediments, typically appearing only as a subtle brownish or greenish shading. The surficial layer of the sediment is a zone of intense microbial and geochemical activity and of considerable physical reworking. In many shallow ecosystems, the biomass of benthic microalgae often exceeds that of the phytoplankton in the overlying waters (McIntyre et al, 1996) such that benthic microalgae play a significant role in system productivity and trophic dynamics, as well as habitat characteristics such as sediment stability.

The predominant macrophyte community of intertidal sand and muds is usually poor unless there are some stones or shells for attachment of species such as Chorda filum, the bootlace weed. The community may include mats of Enteromorpha and Ulva, possibly in large aggregates to form the so-called ‘green tides’ (Piriou, 1991). Seagrasses e.g. Zostera, occur in sheltered sand and mudflats both intertidally and in the shallow subtidal (see Volume I) whilst in sheltered brackish conditions on the upper shore saltmarsh plants such as the cord grass Spartina may become established.

The predominant factor controlling the intertidal community is exposure (Eleftheriou & McIntyre, 1976) and the type of community present ranges from more robust mobile forms in exposed areas to more sensitive sedentary forms in the more sheltered areas. Zonation schemes have been described for macrofauna on sandy intertidal areas, for example Dahl (1952) and Salvat (1964) based zones on the ability of the sediment to retain water. Some species are adapted to exposure to air e.g. Scolelepis squamata and Haustorius arenarius although many species are mobile and migrate to avoid prolonged exposure (McLachlan, 1983).

The meiofauna are likely to be important consumers of the microphytobenthic productivity, yet little is known about meiofauna herbivory in these environments (Montagna, 1995). Intertidal meiofauna, particularly harpacticoids, have a dependent relationship with their autotrophic food resources and can regulate their behaviour to maximise intake of food. However, many aquatic nematodes, which reach high densities in fine particle shores, are opportunistic feeders and may change feeding strategies in response to available food (Moens & Vincx, 1997). Harpacticoid copepods are common to intertidal and subtidal areas. Slender species inhabit the large interstitial spaces found on sandy beaches and larger epibenthic and shallow burrowing forms are more common in fine sediment habitats.

Severe exposure with resulting coarser mobile sands produces low diversity, absence of sedentary forms, especially bivalve molluscs and a dominance of agile swimming forms. These species have a short lifespan and are characterised by their ecological flexibility. This community was classified as the crustacean/polychaete community and in north-west Europe, consists of small, burrowing haustoriid and oedecerotid amphipods and polychaetes (McLachlan, 1996) in which diversity increases towards the low shore area (Eleftheriou & McIntyre 1976). Most fauna may live between mid tide level and the low water mark where Eleftheriou & McIntyre (1976) found crustaceans accounted for 52-98% of the individuals but the polychaetes, because of their greater size, 42-77% of the dry weight.

Table - Typical fauna found in the Biotope Complexes and their Subdivisions

These areas have fine sands which favour the establishment of a predominantly sessile community of polychaetes and long-lived bivalves, restricting swimming forms of amphipods and isopods and some errant polychaetes (Eleftheriou & Holme, 1976). Such areas encourage the colonisation of the intertidal area by subtidal species (denoted (s) in the Table) and the other, intertidal, species follow a zonation pattern. The communities associated with these areas have been described as similar to the Boreal shallow sand association described by Jones (1950) and the Tellina (now Angulus/Fabulina) community. Where the sand mason, Lanice occurs in large numbers on medium sands, it has been described as a separate community (Lanice community).

Sheltered shores are found in areas of low energy and have poorly sorted sediments with high levels of organic matter and an increased silt content (Dyer, 1979). Extreme shelter favours the establishment of a predominantly sessile tube-dwelling community of polychaetes which are often numerically dominant with bivalves also well represented (Atkins, 1983). Some species characteristic of subtidal areas may also occur (see Table). The heart-urchin Echinocardium cordatum occurs in both muddy and clean sands, although it grows much more slowly in the former (Buchanan, 1966).

Estuarine mud flats (low energy areas) have well-defined macrobenthic community (see Table, Figure 3.0) (Jones & Key, 1989; McLusky, 1989) which is similar to the Boreal shallow mud community described by Jones (1950) and also the Scrobicularia community. In addition, several tidal migrants occur including mysids, amphipods and decapods or drifting species associated with algal growths (e.g. Melita obtusata, Dexamine spinosa, Stenothoe marina, Idotea spp.). Often the fauna shows low species diversity, even though biomass may be high, but this depends on the amount of silt present. Many of the species described above for sheltered sandy mud shores will also colonise muddy shores e.g. Arenicola, and on estuarine mudflats enchytraeid and tubificid oligochaetes such as Tubificoides benedeni are often numerically very dominant.

In fully marine areas the organic content is lower and surface deposit-feeding terebellids e.g. Lanice conchilega, and spionid polychaetes and the filter-feeding bivalve Cochlodesma are common. The upper oxygenated layer of sediment extends from between about 3 and 7cm, but the larger tube-dwelling deposit feeding worms such as Rhodine, and the bivalve Thyasira flexuosa, which create extensive feeding channels in the sediment, are normally found in the deoxygenated zone but they extend their respiratory and feeding activities to the surface (Pearson & Eleftheriou, 1981). Firm muds may support piddocks such as Barnea candida and the boring spionid worm Polydora ciliata, while less well-consolidated muds are characterised by other nereid, spionid and capitellid worms.

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