What is Intertidal Zonation?
Intertidal zonation refers to the tendency of plants and animals to form visible communities along the marine shoreline, between the high and low tide lines. The ability of plants and animals to tolerate the special conditions of the intertidal zone determines where on the shoreline they can live. They must be able to survive pounding surf, daily flooding with salt water, exposure to the air, large fluctuations in temperature, and sometimes exposure to fresh water from the rain and seeping groundwater. Starvation is also a risk when the tide is out, since most intertidal animals feed only when they are submerged.
The length of time that organisms are exposed to the air depends on the local tidal range and on where on the shore they are located. Close to the high tide line, plants and animals may be submerged only for an hour or two per day, and must be able to withstand prolonged drying. Near the low tide line, in contrast, they are almost constantly submerged.
Different species have evolved different strategies to cope with the challenges of life in the intertidal zone. For example:
* Mobile species such as crabs hide under rocks, in moist crevices or under seaweeds to escape the sun and keep from drying out.
* Some attached animals like barnacles and mussels can hold a small amount of seawater within their shells, and close up tightly.
* Some algae such as Porphyra, Fucus and Enteromorpha have the remarkable ability to survive losing 60-90% of their moisture, to the point of becoming brittle; when the tide comes back in, they reabsorb water.
* Many species are adapted for holding on against the motion of waves. For example, kelp anchor themselves with holdfasts, and barnacles stick to the rocks by secreting cement.
Plants and animals with similar tolerances to the stresses of intertidal life tend to form communities or bio-bands. These horizontal striations are sometimes easy to spot, particularly on rocky shorelines, due to differences in colour and form. For example, black encrusting algae and lichens may predominate high up on the shore; as one walks toward the water, the rocks suddenly appear covered in barnacles; next to the water, rockweed and mussels are common.
Intertidal zones are generally divided into the following (see the diagram below):
The backshore is the upper area of land between the high tide line and an imaginary boundary 50 metres inland. Terrestrial species like forests and land animals dominate this zone.
Closest to the high tide line, the backshore zone is fringed with the spray zone. This area is not covered by the high tide, but it is exposed to salt spray. It is characterized by periwinkles (a type of snail), black encrusting lichens (Verrucaria) and orange lichen (Caloplaca).
The intertidal zone is the broadest intertidal zone. It lies between the highest high tide and the lowest low tide, and is flooded once or twice daily by the tide. The intertidal zone is marked at its upper limit by barnacles, and closest to the low tide by large kelps (e.g. Laminaria). Other species present include Fucus (rockweed), barnacles, limpets, Nucella (a snail), hermit crabs, mussels, anemones, chitons, sea stars and goose-neck barnacles, to name a few.
The subtidal fringe is exposed only by the lowest of low tides. It borders the subtidal zone, which is always submerged. The subtidal fringe has the richest diversity of species of all the intertidal zones, and contains those that can tolerate only a short exposure to air. Some of the many species in this zone include kelp, eelgrass, red algae, sea slugs, sponges, bryozoans, snails, limpets, crabs, sea anemones, sea stars, sea urchins, sculpins and sea cucumbers. http://www.crd.bc.ca/watersheds/ecosystems/intertidalzonation.htm Rocky Shores “Tide pools are indeed another window to the reality of nature .”
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