Patterns of Zonation on Temperate Rocky Shores

Descriptive studies from the last two centuries describe a wealth of patterns drawn from observations on every continent. While there are clear central tendencies in the zonation of individual species, all show variation. For example, the barnacles Chthamalus fissus on wave-beaten shores of the Pacific Northwest of North America typically occur as a dense band high on the shore, but at some locations the high intertidal band runs parallel to other bands of conspecifics growing as epibionts on mid- to lower-shore species, or as a band of diffuse individuals on the rock surface across a broad range of tides. Similarly, Fucus vesiculosus, a brown alga common on Atlantic shores sheltered from wave action may occur as either a single or double band, bordered above and below by different species, depending on location (Figure 3). Thus, it has not been possible to formulate a universal scheme of zonation based on the details of particular species.

Despite the variation in the resolution of individual species, there is consensus about the generality of a scheme defining three zones based on assemblages of the most conspicuous sedentary species. The latter may form narrower bands within the three broad zones: (1) The zone on highest shore levels, termed littoral fringe or supra littoral zone, consists principally of a thin black film of lichen genera (e.g., Verrucaria spp.) and cyanobac-teria (blue green algae). Animals common in the littoral fringe include small snails (principlly Littorina spp.) and grapsid crabs, chironomid larvae, and isopods, depending on the location. (2) The middle level, termed the eulit-toral or mid-littoral zone, encompasses the majority ofthe tidal range, and is populated by a diverse array of sedentary invertebrates and small algae. The upper reaches of the eulittoral zone are frequently occupied by bands of acorn barnacles (e.g., Semibalanus spp., Chthamalus spp.) and limpets (e.g., Collisella spp., Patella spp.). Toward the lower levels of the eulittoral zone, other suspension feeding invertebrates (see also Suspension Feeders) are conspicuous, often forming bands of individuals several layers deep (e.g. mussels Mytilus spp. or oysters Crassostrea sp.). Algae include Endocladia sp., Mazzaella sp., and Chondrocanthus spp. to name a few. (3) The lowest zone, termed sublittoral fringe or infralittoral zone, is recognized by dense covers of comparatively large brown algae (e.g., Laminaria spp., Egregia sp., Durvillaea sp.) which overlay various algal crusts, fleshy red algae, or articulated coralline algae, any combination of which may extend in varying degrees to below the level of the lowest low tides.

The forgoing descriptions pertained to wave-exposed shores of cold temperate to cool temperate latitudes. Shifts in the species composition, relative widths, and shore levels of the three major zones correlate with numerous environmental factors varying over a range of spatial scales. Correlated factors include latitudinal temperature regimes, mesoscale variation in

(a) British Isles, Southern Norway

Sheltered

Pelvetia

F. spiralis

F. vesiculosus

Ascophyllum

F. serratus

Ascophyllum

F. serratus

Laminaria digitata L. saccharina

(b) New England/Canada (c) New England/Canada

Moderately exposed Sheltered

F. vesiculosus

Ascophyllum

F. vesiculosus

Chondrus

Ascophyllum

F. vesiculosus

Chondrus

F. spiralis

F. vesiculosus

Ascophyllum

F. evanescens

Mastocarpus

Chondrus

F. vesiculosus

Ascophyllum

F. evanescens

Mastocarpus

Chondrus

Figure 3 Diagrammatic representation of the zonation of Fucus spp. and other perennial algae on rocky shores of the north Atlantic. Note the variation in species bordering F. vesiculosus, including the double band of this species in the center panel. Adapted from Raffaelli D and Hawkins SJ (1996) Intertidal Ecology. London: Chapman and Hall.

oceanographic productivity (primary productivity, nutrient availability, and larval supply), the aspect (slope) and composition of the substratum, and exposure to wave action. For example, in warm temperate latitudes of both Africa and South America, aggregations of large ascidians (Pyura spp.) or anemones appear in place of the beds of mussels and large brown algae of cooler latitudes. Differences in the composition of zones between west-and east-facing coasts of the Cape of Good Hope and New Zealand coincide with distinct offshore currents differing with respect to temperature, nutrient concentrations, and meroplankton (larval fauna) composition. Zones ofindividual species are commonly observed to be more numerous and occur at higher shore levels on gradually sloping shores, than on otherwise similar steep shores. The zones shift abruptly up and down or change species composition where vertically stratified rock formations intersect the horizontal bands of the species aggregations.

Apart from tidal variation itself, the strongest and most consistent physical correlate of zonation patterns is the degree of wave action (see Waves as an Ecological Process). Highest wave energies are generated in the open ocean. Coastlines produce alongshore (horizontal) gradients in wave action, and hence gradients in bottom flow speeds and turbulence, in part because wave energy concentrates on oceanward promontories and dissipates to their lee. The scale of the gradients depends on the rate of change of the shoreline contours, and can be quite variable along a coastline. On the wave-beaten sites, the littoral fringe occurs far above the still-water levels of the highest tides, and it is wetted only by wave splash, spray, or precipitation. The upper limits of the three major zones occur at successively lower levels as one proceeds from wave-exposed to sheltered areas. The decline is greatest for the littoral fringe, the upper boundary of which falls within the range of the tides on very sheltered shores, and least for the sublittoral fringe, the upper boundary of which shifts only slightly lower from exposed to sheltered sites. Thus, the three zones appear to compress vertically over the horizontal wave exposure gradient (Figure 4). The alongshore shift in the levels of the eulittoral zone is accompanied by changes in its species composition. The aggregations of limpets and beds of suspension feeders of wave-beaten shores diminish in abundance alongshore until they become sparse populations or absent altogether, and the cover of the eulittoral zone consists most frequently of an overstory of Fucus spp., in some places joined by bands of other algae (e.g., Chondrus sp., Ascophyllum sp.). The understory includes various algal crusts (e.g., Lithothamnion spp. and Ralphsia spp.) and the sparse invertebrate populations.

The forgoing descriptions focused on assemblages of perennials that define the three major zones. Seasonal variation in biotic and physical factors may cause ephemeral

Exposure

Shelter

Exposure

Zonation Brown Algae From Upper Shore

Black lichen/blue green/littorines

Porphyra

Small barnacles

Tetraclita (Large barnacles)

Oysters

Brown algae

(b) New Zealand

Exposure

(b) New Zealand

Exposure

(c) Tropical West Africa

Shelter

Exposure

Black lichen/blue green/littorines

Porphyra

Small barnacles

Tetraclita (Large barnacles)

Oysters

Brown algae

Shelter

Black lichen/blue green/littorines Barnacles

Oysters and apophloae

Hormoisira

Carpophyllum

(c) Tropical West Africa

Black lichen/blue green/littorines Barnacles

Oysters and apophloae

Hormoisira

Carpophyllum

Shelter

Black lichen/blue green/littorines

Barnacles

Mussels

Lithothamnia

Red algal turf

Sea urchins

(d) New South Wales, Australia

Shelter

Exposure

(e) British Isles

Exposure

Black lichen/blue green/littorines

Barnacles

Mussels

Lithothamnia

Red algal turf

Sea urchins

Shelter

Exposure

Chthamalus Zonation

Black lichen/blue green/littorines

Chthamalus

Tessepora

Coralline Turf

Black lichen/blue green/littorines Porphyra

Barnacles and limpets

Mussels

Fucoids

L. hyperborea

L. digitata L. saccharina

(f) General

Black lichen/blue green/littorines

Chthamalus

Tessepora

Coralline Turf

Black lichen/blue green/littorines Porphyra

Barnacles and limpets

Mussels

Fucoids

L. hyperborea

L. digitata L. saccharina

(f) General

Exposure

Shelter

Shelter

Figure 4 Diagrammatic representation of the vertical compression of the major zones alongshore from exposed to sheltered extremes of the wave energy gradient. Widely separated regions in temperate latitudes show similar patterns, though exceptions can be found in any particular region. Adapted from Raffaelli D and Hawkins SJ (1996) IntertidalEcology. London: Chapman and Hall.

increases in certain species, and violent physical disturbances (drift log impacts, ice scour, sediment inundations, etc.) may occasionally obliterate zones, initiating a succession of subdominant species (see Succession).

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Responses

  • Teresa Baresi
    Why do widely seperated rocky shores show similar oatterns of zonation?
    7 years ago
  • Arto
    What is population zonation?
    7 years ago
  • maria hertzog
    Does carpophyllum show a pattern of clumping?
    7 years ago

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