The intertidal zone or littoral zone is that part of the seabed that lies between the highest high water mark (extreme high water spring level or EHWS) and the lowest low water mark (extreme low water spring level or ELWS). It is sometimes convenient to refer to various levels of the shore in terms of the tidal cycle, as indicated in Figure 8.3. The strip of shore between mean high water level and mean low water level has been termed the middle shore, above MHW the upper shore and below MLW the lower shore.
The distribution of plants and animals on the shore is sometimes described in terms of the standard tidal levels but this can be misleading because, as discussed later (see Section 8.6), many other factors besides the tidal cycle influence the levels occupied by shore organisms, and their distribution varies from place to place. There is also some difficulty in determining the tidal levels with accuracy. Nevertheless, this terminology does provide a useful means of giving a general indication of the vertical zonation of different species in particular localities.
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