California's Rocky Intertidal Zones

The following materials are exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.

Between the high and low tide marks lies a strip of shoreline that is regularly covered and uncovered by the advance and retreat of the tides. This meeting ground between land and sea is called the intertidal. The plants and animals inhabiting this region are hardy and adaptable, able to withstand periodic exposure to air and the force of the pounding surf. Intertidal communities occur on sandy beaches, in bays and estuaries, and on wharf pilings, but the communities of rocky shorelines are perhaps the most diverse and the most densely populated. Rock faces, crevices, undersides of rocks, and tidepools each support and array of species.

The plants and animals of the intertidal are subject to a range of conditions not encountered in the relative stability of the deep ocean. Three factors--substrate, wave shock, and exposure to drying--are important in determining the types of organisms found in a given intertidal community. Soft substrates, such as sandy beaches and mudflats, support an abundance of burrowing animals, whereas sessile, or attached, organisms are more typical of rocky shores. The surf has the potential to batter of dislodge plants and animals. Some areas of the coast, particularly rocky headlands and exposed outer coasts, experience tremendous wave action; here only the most tenacious organisms survive. Sheltered embayments, and coastal areas protected by offshore rocks, reefs, or islands, receive considerably less wave shock, and support a variety of more delicate forms, However, the ability to withstand the desiccation and overheating while exposed to air by low tides may be the most important facto in determining where marine organisms occur in the intertidal. The extent to which and organism is exposed to air is largely determined by its vertical position in the intertidal region, and the pattern of the tides.

The tides are generated by the gravitational forces of the moon, and, to a lesser extent of the sun. Because these astronomical motions are very regular, tides are highly predictable. Gravitational forces attract the earth and moon to each other, whereas centrifugal forces keep them separate. One the side of the earth closest to the moon, the moon's gravitational pull is stronger than the centrifugal force, and causes the earth's waters on this side to bulge out. On the opposite side of the earth, centrifugal forces override the moon's gravitational force, and result in a second, equal bulge of water. Between these two bulges are corresponding areas of low water.

In some coastal areas the two daily high tides and low tides are of nearly equal magnitude. On the California coast, the pairs of high and low tides differ in magnitude, so that there is a higher high tide, followed by a lower low tide, a lower high tide, and a higher low tide each day. These mixed tides are caused by the moon and sun's changing position with respect to the earth's equator. If the moon and the sun were always directly over the equator, the two daily high tides experienced at any one place would be equal in magnitude. But because the earth is tipped on its axis, as the moon orbits around the earth it moves higher and lower in the sky relative to the equator twice each month, thus shifting the position of the tidal bulges. Similarly, as the earth orbits around the sun, the sun moves higher and lower in the sky twice each year.

Tides of maximum range, called tropic tides, occur twice a month when the moon is over the Tropic of Cancer or the Tropic of Capricorn. Tropic tides are particularly large in summer and winter when the sun is respectively highest and lowest in the sky. Tides of minimum range, or equatorial tides, occur twice each month when the moon is over the equator. Tides are the largest single factor contributing to sea level changes in California. Other influences such as storms, warming of the ocean water, and El Nino conditions raise the sea temporarily, and so far are not predictable.

Intertidal plants and animals tend to occur in distinct bands, or zones, along the shore. Where organisms occur in the intertidal depends upon their varying responses to exposure to air, heat, and sunlight, predation or grazing, and competition for space. The intertidal can be divided into four zones according to the extent of tidal exposure; the uppermost horizon, and the high, middle, and low intertidal zones. The exact tidal level at which these zones occur on a given shore varies; wave action tend to widen the zones, whereas in areas of quiet water, the zones are more narrow.

Rocky intertidal areas can be found all along the California coast; they are best observed at the lowest tides. Some good places to observe intertidal life include Trinidad Sate Beach in Humboldt County, Shell Beach in Sonoma County, Fitzgerald Marine Reserve in San Mateo County, Point Pinos in Monterey County, Crystal Cove State Park in Orange County, and Children's Pool Beach in San Diego.

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