The Vertical Emersion Gradient

The Horizontal Wave-Action Gradient

The Interaction Between Emersion and Wave Action

On gently shelving rocky shores, the littoral zone may extend for hundreds of metres. However, rocky shores are often steeply shelving and the littoral zone therefore occupies a few tens of metres or less. The interface between air and water along with the action of tides and waves result in a vertical emersion gradient of increasing exposure to air from low shore to high shore. A horizontal gradient of exposure to wave action also exists both among microhabitats within shores and among different shores. The interaction between these gradients is of prime importance in determining the type of organisms that any area of hard shore will support. Clear, well studied, patterns of zonation of flora and fauna exist on rocky shores as a result (Stephenson and Stephenson, 1972; Lewis, 1964; Ballantine, 1961).

Compared with subtidal habitats, conditions fluctuate much more in the littoral zone. The sea provides a buffer, damping temperature fluctuations and maintaining a steady concentration of salts and nutrients. At the interface between land and water, organisms spend part of their time immersed in the sea, or at least splashed by its spray, and part of their time in contact with the air, with a vertical gradient of emersion up the shore. Since most littoral organisms are of marine origin, this results in a unidirectional stress gradient. Air temperatures commonly fluctuate by 10 to 20° C in a single 24 hour period whereas sea temperatures usually fluctuate by less than 10° C in a year. In polar and boreal regions, intertidal air temperatures might fall as low as - 40° C while the sea is never colder than about -0.2° C. Salinity in pools and crevices in the littoral zone can vary considerably with evaporation and dilution by rain. Water-borne food, nutrients, dissolved oxygen and the protection from desiccation and extreme temperatures conferred by immersion, are available for less of the time at increasing shore heights. In temperate zones, the risk of desiccation due to heat and low humidity is the most significant stress. Extreme cold is a more important stress at higher latitudes. A gradient of increasing stress to terrestrial organisms due to immersion runs down the shore. These vertical stress gradients have important ecological effects on rocky shores, contributing to the clear zonation patterns within the biological community.

Tidal ranges vary from 0.5m to 12m in the British Isles. Greater tidal ranges result in more extensive littoral zones. However, even in the absence of tides, a zone exists in which the sea laps against the shore or waves break and splash (the ‘splash zone’). The form of tides also varies along the U.K. coast: most are semi-diurnal with two low tides and two high tides every 24. hours. In some parts of the British coast, particularly in the region between Portsmouth and the Isle of Wight, double low and double high waters can occur resulting in very low water stands on spring tides

The time of low water varies with geographical position. This will determine the type of stress suffered by organisms when exposed. This is particularly relevant to low shore species which rely on the sea to protect them from environmental extremes as much as possible. Desiccation stress will be most severe when spring tide low water occurs at around midday, particularly if exaggerated by a double. The geographical range of species can be limited by currents. The sea urchin, Paracentrotus lividus occurs on the West coast of Ireland but does not establish on shores in England or Wales. Conversely, Patella depressa is common around southern England and Wales but absent from Ireland (Southward and Crisp, 1955).

The structure of ecological communities on rocky shores is also affected by a horizontal gradient of exposure to wave action, from sheltered bays to exposed headlands. The extent of wave action on a particular shore is determined by the aspect to prevailing winds coupled with the ‘fetch’: the distance over which winds blow. Shores with a long fetch have heavy wave action because the wind has a greater distance to generate the height of the waves. Such shores may also receive swell on windless days, resulting from distant storms. Oceanic islands and headlands on ocean coasts therefore have the most exposed shores. Bays in enclosed water bodies such as sea lochs and the Irish Sea will experience the least wave action, especially when they face away from the prevailing wind. The list of candidate SACs for the UK includes rocky shores at both very exposed (St. Kilda, Papa Stour, Lundy) and very sheltered (Loch Duich, Loch Alsh, Loch Long) sites.

Exposure to wave action affects the distribution of organisms. Increasing exposure reduces siltation and increases the supply of dissolved oxygen and particulate food, favouring certain sessile, filter-feeding species. At the same time, increasing exposure carries an increased risk of dislodgement and physical damage, limiting the range of susceptible and physically fragile species. Morphological differences can be observed between members of the same species from wave-exposed and sheltered sites. For example, dogwhelks from wave-exposed shores have thinner shells with larger apertures than those from sheltered shores. Wave-exposed morphs are better at clinging to the substratum while sheltered morphs are better protected against predation by crabs.

In contrast to the unidirectional nature of the vertical emersion gradient, the horizontal gradient is less well defined: some species do well on wave-exposed shores, some do best in shelter and others under intermediate conditions. However, the degree of exposure or shelter required by a species may vary along its geographical range. For example, the limpet Patella vulgata is restricted to wave-exposed shores in Norway and sheltered shores in Spain while in Britain it is found on both shore types.

Each of the two gradients described above has its own effect on community structure. The degree of exposure to wave action can modify the extent of the vertical gradient. As wave action increases, so too does the amount of spray produced. Waves with greater amplitude break at higher shore levels, showering still higher areas with spray. At very exposed sites, shore levels well above the highest tide level may be regularly wetted by the action of waves. It is not uncommon to find high shore species many tens of metres above the theoretical tidal limit on very exposed cliffs (Lewis, 1964). Species which are referred to as intertidal may therefore exist above the theoretical extent of high water springs on a calm day. Any useful assessment of the shore community must include these organisms.

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