Physical factors affecting distribution of communities

We have already outlined in Chapter 4 the major hydrographic parameters which control the distribution of marine organisms - the temperature of the water, its composition, movements, pressure and illumination. Except on the shore and in shallow water, these vary less at the bottom than they do in the upper levels of water. They are none the less important in relation to the distribution of benthic populations, restricting certain species to particular localities and often having a major effect during the early stages of life when the majority of benthic creatures pass through pelagic phases. The planktonic eggs and larvae of many species are highly susceptible to the quality of the water in which they float, and the period prior to completion of metamorphosis is always the time of heaviest mortality. Predation and shortage of suitable food exact a heavy toll, and losses are specially severe if water temperature or salinity is unfavourable.

Substrate

The substrate material exerts a dominant influence over the distribution of organisms on the sea floor. Where the bottom is rock or large stones, the community consists chiefly of forms which live on the surface of the substrate, i.e. it is an epifauna and epiflora. The animals are mainly sessile or encrusting cnidarians (coelenterates), sponges, bryozoans, barnacles, tubicolous worms, mussels and sea squirts. Crawling among them are a variety of errant polychaetes, starfish, echinoids, gastropods and large crustaceans such as crabs, lobsters and crawfish. In shallow water where sufficient illumination reaches the bottom, seaweeds grow attached to rock or to stones heavy enough to give secure anchorage, and to each other. There is usually a wide diversity of species inhabiting a rocky bottom because the irregularities of the rock surface provide a great variety of microhabitats, with innumerable differences of living space, water movement, food supply, illumination and temperature. A rocky bottom does not support a numerous infauna, i.e. animals dwelling within the substrate, but burrowing creatures occur in accumulations of silt in rock crevices. There are also a few forms capable of boring into rock, mainly bivalve molluscs (Hiatella, Pholas, Lithophaga), a few annelids (Polydora, Dodecaceria), the sponge Cliona and certain barnacles and sea urchins. In some areas there are also species of red algae which bore superficially in calcareous rock.

Where the bottom is covered with sediment, most of the inhabitants live within the deposit. Local conditions are generally more uniform and communities less diverse than on a rocky bottom. The infauna includes burrowing sea anemones, polychaetes, bivalves, gastropods, echinoderms and crustacea. Some fish also burrow superficially in the deposit and a few, such as the red band fish (Cepola) make deep burrows. The particle size of a sediment is an important factor regulating the distribution of the infauna because the mode of burrowing of many creatures is specialized and suitable only for a certain grade of substrate (Trueman and Ansell, 1969). Burrowing can be done by forcing or digging through the sediment, pushing the particles aside, or eating through it, or often by a combination of methods. Large particles are more difficult to displace or ingest than small ones, and the mechanical difficulty of burrowing in coarse deposits may be one reason why these are usually less populated than finer ones. On the other hand, very fine sediment can compact into a dense, unyielding mass in which it is not easy to burrow and which requires adaptations for dealing with silt. Certain combinations of particles form thixotropic deposits which are readily reduced to a semi-fluid consistency by repeated, intermittent pressures and yield easily to burrowing. Where the deposit is exceptionally soft, such as occurs beneath some areas of very deep water, animals may simply sink into it, and here we find adaptations such as stalks or extremely long appendages to lift the main body clear of the bottom.

Although differences between communities can often be correlated with differences of particle size of sediments, other factors are also involved. For example, many sediment-dwelling animals do not actively burrow, and yet may be quite particular in their choice of sediments. The rather similar common British sea cucumbers Neopentadactyla mixta and Thyone fuscus live in quite different sediments, Neopentadactyla in coarse gravels and Thyone in mud. Here the size of sediment particles is unlikely to be the only factor controlling their distribution. The grade of a deposit depends upon the speed of bottom current, and this also controls several other features of the substrate. Slow-moving water allows organic matter to settle, giving a sediment that may be not only fine in texture but also rich in organic content. Poor or absent circulation of the contained water leads to deficient oxygenation of the subsurface layers, and high concentrations of sulphide. Beneath shallow water these conditions often support a large biomass because there is a good food supply for creatures which feed on the surface or digest organic matter from the sediment, but the infauna must be able to cope with silt and a deoxygenated medium. Where the bottom water moves more swiftly there is likely to be less settlement of food and a lower organic content in coarser sediments, but better oxygenation of the interstitial water. The poorer food supply supports a smaller biomass, but these conditions favour animals which can burrow in coarse material and capture floating food suspended in the water. Therefore, several interrelated factors must operate to limit certain species to particular substrates.

Turbidity

The turbidity of the water is another factor to which some benthic organisms are very sensitive. The quantity of suspended matter is often considerably greater in water close to the bottom than in layers nearer the surface. In shallow water this reduces illumination and may therefore restrict the distribution of benthic plants. High turbidity may also have adverse effects on animals by clogging the feeding apparatus or smothering the respiratory surfaces. Many benthic creatures are filter-feeders, notably the lamellibranchs which form a major part of the community in many sediments. These obtain food by drawing in a current of water from which they filter suspended food particles, and special adaptations are required to cope with the problem of separating food from large quantities of silt. Turbidity is also related to wave exposure and water currents. Shallow sheltered lochs or bays are likely to have poorer water clarity than similar but more open areas. Water movement carries settling silt away and prevents it from accumulating and smothering the benthos.

Water currents

Benthic organisms are also influenced by the speed of the bottom current because this controls the particle size of the substrate, its oxygenation and organic content, and also affects the dispersal of pelagic larvae and the ease with which they can settle on the bottom. The bottom current is also important in the transport of food particles, sweeping them away from some areas and concentrating them in others, especially in depressions in the sea-bed.

Pressure

At abyssal depths, where temperature and salinity are uniform over great areas, hydrostatic pressure may well be the chief factor which accounts for differences between communities within the ocean trenches and those of other parts of the deep-sea bottom.

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