Intertidal Zone

The intertidal zone, also sometimes referred to as the littoral or eulittoral zone, refers to the part of the marine, benthic environment (see Benthos) between the maximum high and minimum low tide levels. The width of this zone may vary tremendously—from a few centimeters to kilometers—depending on both the amount of tidal exchange and the underlying slope of the shore. In addition, the character of the intertidal zone will depend on whether the shoreline is composed of muddy or marshy embayments, sandy beaches, mangroves, or rocky bedrock, boulders, or walls. These diverse intertidal zones all play important ecological roles as habitats in their own right, as well as being a series of transitional "ecotone" habitats between true terrestrial and true subtidal areas. What unites these disparate environments is the periodic cycling between exposed, terrestrial conditions and submerged, marine conditions.

Because intertidal organisms may spend portions of their daily lives exposed to both

An intertidal zone with kelp beds, South Africa (Gallo Images/Corbis)

marine and terrestrial environmental factors— such as direct solar radiation, wind, waves, and water with a wide range of salinities—they must be more physiologically tolerant than most marine organisms. The degree of environmental tolerance will determine in part the portion of the intertidal zone that individual species can inhabit. For example, those that are especially resistant to heat and desiccation may live higher in the zone in the upper intertidal, while those that are more susceptible to those stresses will be limited to lower levels (that is, the lower intertidal).

In addition, the ecological processes of competition and predation will also influence which parts of the intertidal zone various species inhabit. Stress-tolerant but competitively inferior species, for example, may be forced to occupy higher, less preferable levels of the intertidal when dominant competitors are present. Similarly, the tidal range of predators can also cause the range of their prey to shift within the intertidal zone. The combined action of physical (that is, abiotic) and biotic pressures may contribute to the pat-terns—both subtle and striking—of species zonation that are frequently apparent in many intertidal habitats.

The presence of strong, consistent environmental gradients across short tidal distances—combined with other environmental gradients, such as the degree of wave exposure—have made intertidal zones extremely important ecological systems for sci entists. Temperate, rocky intertidal habitats containing many fixed (sessile) or slow-moving organisms (such as mussels, barnacles, rockweeds, and kelps) that must compete for limited space within their narrow, preferred ranges in the overall intertidal zone, have been particularly important to ecological research.

—Daniel Brumbaugh

See also: Ecological Niches; Ecology; Ecosytems; Oceans

Bibliography

Carefoot, Thomas. 1977. Pacific Seashores: A Guide to Intertidal Ecology. Seattle: University of Washington Press; Ricketts, E. F., J. Calvin, and J. W. Hedg-peth. 1985. Between Pacific Tides, 5th ed. Stanford: Stanford University Press; Stephenson, T. A., and A. Stephenson. 1972. Life between Tidemarks on Rocky Shores. San Francisco: W. H. Freeman.

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