Production and Phytobenthos
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
Benthic Fauna in Relation to Exposure
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
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
Moderately exposed shores
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
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.