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Vertical Zonation and the Rocky Shore
According to Yonge (1949), “the zoning of both plants and animals within the narrow vertical limits of a rocky shore is one of the most striking features about shore life.” It is noticeable particularly where sessile organisms (such as barnacles) occur in abundance on the boundaries of their vertical distribution, as they form a conspicuous line (Boaden & Seed, 1985). The author has also been particularly impressed by clear zonation patterns of brown algae often to be found on rocky shores. Precise and universal demarcation of these zones is impossible (Brehaut, 1982), but a comprehension of the vertical zonation of rocky shores is clearly essential to an understanding of their ecology.
The first attempts to differentiate zones were based on tidal classification (Boaden & Seed, 1985). It is easy to distinguish (Brehaut, 1982) a sub-littoral zone (one which is never completely uncovered by water), a supra-littoral zone (never completely covered), and a littoral zone (where periodic covering and uncovering by water is normal). These are little more than terms of convenience however, as the extreme high and low levels of tide may only be reached at intervals a considerable number of years apart. Since many organisms that dwell on rocky shores are annuals, this is of no significance to them.
Although zonation is related to tides, it is influenced by other factors (discussed below). Indeed, Stephenson and Stephenson (1972) note, “Zonation, although undoubtedly related to tides, is not directly caused by them.” Lewis (1964) emphasises that restricting zones to tidal boundaries causes confusing terminology. The littoral zone should not be defined in relation to sea level, but rather as a strip of the shore which undergoes alternating periods of exposure to air and submersion under seawater, and where characteristic communities of organisms thrive.
Stephenson and Stephenson’s scheme of classification, pioneered in the 1940s, was revolutionary because it was the first to use biological, rather than physical, criteria to define the major zones. They found that widespread, universal distribution factors could be used to differentiate discrete zones.
Lewis (1964) modified this scheme, recognising three distinct zones: the littoral fringe, the eulittoral zone, and the sublittoral zone. The littoral zone, made up of the littoral fringe and the eulittoral zone, is not related to the physical description of the shore; but is a biologically defined area.
The ‘widespread, universal’ features shared by many rocky coasts around the world were noticed by Stephenson and Stephenson (1972). They found, firstly, that near the high-water level there is an arid zone, the littoral fringe. This zone is transitional between land and sea, and as such is subject to conditions common to both. It is affected by spray, but is only rarely wetted entirely by waves (in rough weather or at extreme high tides). As Tait and Dipper (1998) note, the littoral fringe varies according to the level of shelter afforded. On relatively sheltered shores the littoral fringe is a narrow strip found almost entirely below the extreme high water spring tide mark. On more exposed shores, however, the zone is modified by wave action and is noticeably higher and wider. The most exposed shores have a littoral fringe which can be up to 20 metres wide, and almost entirely above the level of extreme high water spring tide.
A relatively small number of species inhabit this zone, and they are dominated by species adapted to arid conditions, such as small liuttorinid snails, and small blue-green prokaryotes. Rock surface in this zone is commonly encrusted with myxophyceans, or Verrucaria lichens, or both. These species blacken the rocks, and this black zone (Stephenson & Stephenson, 1972) is “one of the most persistent features of shores.”...
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