Conditions at the boundary between the sea and land

Salinity has important effects on the distribution of organisms in intertidal areas but it does so through interactions with other conditions - notably exposure to the air and the nature of the substrate.

Figure 2.18 Standard metabolic expenditure (estimated through minimum oxygen consumption) in two species of shrimp, Palaemonetes pugio and P. vulgaris, at a range of salinities. There was significant mortality of both species over the experimental period at 0.5 ppt (parts per thousand), especially in P. vulgaris (75% compared with 25%). (After Rowe, 2002.)

Figure 2.18 Standard metabolic expenditure (estimated through minimum oxygen consumption) in two species of shrimp, Palaemonetes pugio and P. vulgaris, at a range of salinities. There was significant mortality of both species over the experimental period at 0.5 ppt (parts per thousand), especially in P. vulgaris (75% compared with 25%). (After Rowe, 2002.)

Algae of all types have found suitable habitats permanently immersed in the sea, but permanently submerged higher plants are almost completely absent. This is a striking contrast with submerged freshwater habitats where a variety of flowering plants have a conspicuous role. The main reason seems to be that higher plants require a substrate in which their roots can find anchorage. Large marine algae, which are continuously submerged except at extremely low tides, largely take their place in marine communities. These do not have roots but attach themselves to rocks by specialized 'holdfasts'. They are excluded from regions where the substrates are soft and holdfasts cannot 'hold fast'. It is in such regions that the few truly marine flowering plants, for example sea grasses such as Zostera and Posidonia, form submerged communities that support complex animal communities.

Most species of higher plants that root in seawater have leaves and shoots that are exposed to the atmosphere for a large part of the tidal cycle, such as mangroves, species of the grass genus Spartina and extreme halo-phytes such as species of Salicornia that have aerial shoots but whose roots are exposed to the full salinity of seawater. Where there is a stable substrate in which plants can root, communities of flowering plants may extend right through the intertidal zone in a continuum extending from those continuously immersed in full-strength seawater (like the sea grasses) through to totally nonsaline conditions. Salt marshes, in particular, encompass a range of salt concentrations running from full-strength seawater down to totally nonsaline conditions.

Higher plants are absent from intertidal rocky sea shores except where pockets of soft substrate may have formed in crevices. Instead, such habitats are dominated by the algae, which give way to lichens at and above the high tide level where the exposure to desiccation is highest. The plants and animals that live on rocky sea shores are influenced by environmental conditions in a very profound and often particularly obvious way by the extent to which they tolerate exposure to the aerial environment and the forces of waves and storms. This expresses itself in the zonation of the organisms, with different species at different heights up the shore (Figure 2.19).

The extent of the intertidal zone zonation depends on the height of tides and the slope of the shore. Away from the shore, the tidal rise and fall are rarely greater than 1 m, but closer to shore, the shape of the land mass can funnel the ebb and flow of the water to produce extraordinary spring tidal ranges of, for example, nearly 20 m in the Bay of Fundy (between Nova Scotia and New Brunswick, Canada). In contrast, the shores of the Mediterranean Sea algae and higher plants

Land

Figure 2.19 A general zonation scheme for the seashore determined by relative lengths of exposure to the air and to the action of waves. (After Raffaelli & Hawkins, 1996.)

Supralittoral fringe

Land

Supralittoral fringe

Midlittoral zone

Midlittoral zone

Jnfralittoral fringe t Infralittoral zone

Sea experience scarcely any tidal range. On steep shores and rocky cliffs the intertidal zone is very short and zonation is compressed.

To talk of 'zonation as a result of exposure', however, is to oversimplify the matter greatly (Raffaelli & Hawkins, 1996). In the first place, 'exposure' can mean a variety, or a combination of, many different things: desiccation, extremes of temperature, changes in salinity, excessive illumination and the sheer physical forces of pounding waves and storms (to which we turn in Section 2.7). Furthermore, 'exposure' only really explains the upper limits of these essentially marine species, and yet zonation depends on them having lower limits too. For some species there can be too little exposure in the lower zones. For instance, green algae would be starved of blue and especially red light if they were submerged for long periods too low down the shore. For many other species though, a lower limit to distribution is set by competition and predation (see, for example, the discussion in Paine, 1994). The seaweed Fucus spiralis will readily extend lower down the shore than usual in Great Britain whenever other competing midshore fucoid seaweeds are scarce.

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