The Universal Features of Zonation Between Tide-Marks on Rocky Coasts

Abstract

On the basis of 30 hrs. of first-hand experience of rocky coasts in Britian, N. America, S. Africa, the Indian Ocean and Australia, it is suggested that certain features of zonation between tidemarks are of such widespread occurrence in the world that they may even be universal. The terminology applicable to the widespread zones is discussed, and a scheme is proposed which defines them and attempts to name them appropriately. The zonation of open rock-surfaces exposed to degrees of wave-action intermediate between max. and min. is regarded as the standard from which deviations may most conveniently be recognized. From above downwards shores tend to show the following belts: (1) Supra-littoral Zone. The maritime belt lying near the sea, above tide-marks, but subject to some maritime influence (e.g., to finely divided spray in rough weather). The lower limit of this zone is the same as the upper limit of the one below. Its upper limit is now within the scope of this paper. (2) Supralittoral Fringe. From the upper limit of barnacles (in quantity) to the nearest convenient landmark above this (e.g., the upper limit of Littorinae or the lower limit of maritime land-lichens or flowering plants). High water of spring tides invades at least the lower part of this zone. (3) Midlittoral Zone. From the upper limit of barnacles (in quantity) down to the upper limit of the zone below. This belt tends to be covered and uncovered every day, at least in part. (4) Infralittoral Fringe. From the upper limit of any convenient dominant organism (e.g., Laminaria, Pyura) to extreme low-water level of spring tides, or to the lowest level ever visible between waves. This zone uncovers only at the major tides, and sometimes only in calm weather. (5) Infralittoral Zone. From extreme low water of springs to a depth which has yet to be settled[long dash]it may be to the edge of the continental shelf or to the lower limit of seaweed vegetation. An account is given of (a) the standard populations of these zones; (b) of some of the principal variations in these populations; (c) of the existence of additional local zones; and (d) of the local absence of ecologically important spp. The terms littoral, supralittoral and infralittoral are discussed, in relation to others, and "littoral" is defined as the equivalent of "intertidal." It is maintained that within the framework of the principal zones, subzones and patch-forming communities, which are much more local in occurrence, can be suitably named, as heretofore, from the organisms that dominate them. The relationship between the zones and tide-levels is discussed, especially to emphasize that the zones cannot be defined in terms of tidal levels although related to these, but must be defined in terms of the distr. of organisms.