The main macrofaunal groups encountered in soft substrata are: Class Polychaete, Subphylum Crustacea, Phylum Echinodermata and Phylum Mollusca. We discussed this in the context of the work done at Millport on an intertidal sandy environment but in our discussed we included also intertidal muddy and sublittoral systems.
We reviewed the classification of benthic animals according to their size (microfauna - <100 µm; meiofauna - 100-500 µm; and macrofauna - >500 µm), and according to where they live (epifauna, on the sediment, or infauna, within the sediment).
The pioneering work of Petersen (early 1920s) and of Thorson (1950s) were discussed. The concepts of infaunal associations (groups of organisms that co-occur) and parallel communities (communities of similar organisms found at similar depths and sediments in the different oceans, particularly in the temperate latitudes) were introduced and exemplified. Although this pattern does not exist, in the same way, in tropical areas, it is likely that these associations are not random but interact (between the organisms themselves and the habitat) in such a way that they tend to persist.
Organisms are distributed along environmental gradients according to their ecological preferences and requirements. The main parameters in coastal areas are: shore level (intertidal - from low to high); exposure (from sheltered to exposed); grain size (from mud to bedrock); and depth (subtidal).
Sediment granulometry was discussed previously and details provided within the Practicals booklet. Physical differences between coarse-grained and fine sediment beaches were highlighted. In a nutshell, coarse-grained beaches will have large interstitial spaces, good water drainage and aeration, low organic matter and no anoxic zone present (or, if present, at deeper levels). For these reasons, fine-grained sediments tend to harbour higher levels of organisms.
Within the benthos all functional groups can be encountered. Sediment primary producers are small microalgae that can be encountered within the first few millimetres of the sediment surface. Diatoms are the main primary producers in sedimentary environments and are mostly found in fine-grained sediments. However, the food web in these environments are mostly based on detritus.
Therefore, at the bottom of the food web there are the planktonic/benthic primary producers and detritus. The second level will be made up by deposit feeders (organisms that acquire their energy from deposited organic matter) and suspension feeders (organisms that acquire their energy from matter in suspension or seston). In both cases, organisms take a combination of live organisms and dead material. The third level is composed by the carnivores and in this category are included a wide variety of organisms, for instances, Polychaete (e.g. the catworm, Nephtys), snails (e.g. whelks), crabs (e.g. shore/green crab Carcinus maenas), fish (e.g. flounder, Platichthys flesus), birds (e.g. plover, sandpiper, dunlin, red shank).
Infaunal organisms either move through the sediment, to capture their prey or to swallow large quantities of organic deposits, or else tend to stay still and capture their food either from the seston (suspension feeders) or from within the sediment while inside their burrows (e.g. lugworm, Arenicola marina). Organisms that move within the sediment or somehow disturb the sediment (i.e. cause it to move, resuspend, erode, redeposit, or somehow, change) during their feeding are called bioturbators (or sediment destabilisers). These include both sessile (organisms that donÕt move) or motile organisms and examples include bivalves (e.g. Macoma, who can alternate from deposit to suspension-feeding), crustaceans (the mud shrimp, Corophium volutator), Polychaete (e.g. lugworm), echinoderms (e.g. the deposit-feeder sea cucumber, Molpadia).
Other organisms, like tube-building Polychaete (e.g. sand-mason Lanice conchilega) tend to consolidate the sedimentary habitat and are therefore called sediment stabilisers. Often assemblages are studied by classifying organisms into functional groups. These can be defined in different ways, but normally include all the species of different animal taxa that use and affect the environment in similar ways. Above we used this type of classification to group organisms into primary producers, consumers, deposit-feeders, suspension-feeders and carnivores. However, as described above, organisms can also be classified as sediment-stabilisers or sediment-destabilisers, and both categories include deposit-feeders, suspension-feeders and carnivores.
Burrowing deposit-feeders tend to be more abundant in fine-grained sediments, where there are higher levels of organic matter and sediments are less mobile. The activity of these organisms tend to create resuspension of fine particles, which in turn, tend to clog the fine filtering structures of some of the suspension-feeders, making their feeding very difficult. In addition, the constant reworking of sediment leads to the burial of the newly settled larvae of the suspension-feeders making their survival very difficult. The larvae of deposit-feeders are not so affected since they tend to burrow deeper and are generally less sensitive. The exclusion of one trophic group by modification of the environment by another was reported by Rhoads and Young in the 1970s who called it trophic group amensalism. In this way deposit-feeders are separated spatially by suspension-feeders. In turn, these tend to be more dominant in coarse-grained environments. This knowledge is often used in pollution monitoring. Since areas impacted by organic enrichment tend to have higher levels of fine-grained sediments, it is expected that these will be dominated by deposit-feeders, whereas ÔcleanÕ areas will be dominated by suspension-feeders. Of course this is strongly dependent on the natural characteristics of the habitat under study.