Intertidal zone

Tide pools at Pillar Point showing zonation on the edge of the rock ledge

A rock, seen at low tide, exhibiting typical intertidal zonation, Kalaloch, Washington, western United States.

Marine biologists divide the intertidal region into three zones (low, middle, and high), based on the overall average exposure of the zone.[1] The low intertidal zone, which borders on the shallow subtidal zone, is only exposed to air at the lowest of low tides and is primarily marine in character. The mid intertidal zone is regularly exposed and submerged by average tides. The high intertidal zone is only covered by the highest of the high tides, and spends much of its time as terrestrial habitat. The high intertidal zone borders on the splash zone (the region above the highest still-tide level, but which receives wave splash). On shores exposed to heavy wave action, the intertidal zone will be influenced by waves, as the spray from breaking waves will extend the intertidal zone.

Depending on the substratum and topography of the shore, additional features may be noticed. On rocky shores, tide pools form in depressions that fill with water as the tide rises. Under certain conditions, such as those at Morecambe Bay, quicksand may form.[2]

This subregion is mostly submerged – it is only exposed at the point of low tide and for a longer period of time during extremely low tides. This area is teeming with life;[1] the most notable difference between this subregion and the other three is that there is much more marine vegetation, especially seaweeds. There is also a great biodiversity. Organisms in this zone generally are not well adapted to periods of dryness and temperature extremes. Some of the organisms in this area are abalone, sea anemones, brown seaweed, chitons, crabs, green algae, hydroids, isopods, limpets, mussels, nudibranchs, sculpin, sea cucumber, sea lettuce, sea palms, starfish, sea urchins, shrimp, snails, sponges, surf grass, tube worms, and whelks. Creatures in this area can grow to larger sizes because there is more available energy in the localized ecosystem. Also, marine vegetation can grow to much greater sizes than in the other three intertidal subregions due to the better water coverage. The water is shallow enough to allow plenty of sunlight to reach the vegetation to allow substantial photosynthetic activity, and the salinity is at almost normal levels. This area is also protected from large predators such as fish because of the wave action and the relatively shallow water.

A California tide pool in the low tide zone

The intertidal region is an important model system for the study of ecology, especially on wave-swept rocky shores. The region contains a high diversity of species, and the zonation created by the tides causes species ranges to be compressed into very narrow bands. This makes it relatively simple to study species across their entire cross-shore range, something that can be extremely difficult in, for instance, terrestrial habitats that can stretch thousands of kilometres. Communities on wave-swept shores also have high turnover due to disturbance, so it is possible to watch ecological succession over years rather than decades.

The burrowing invertebrates that make up large portions of sandy beach ecosystems are known to travel relatively great distances in cross-shore directions as beaches change on the order of days, semilunar cycles, seasons, or years.[3] The distribution of some species has been found to correlate strongly with geomorphic datums such as the high tide strand and the water table outcrop.[3]

Since the foreshore is alternately covered by the sea and exposed to the air, organisms living in this environment must have adaptions for both wet and dry conditions. Hazards include being smashed or carried away by rough waves, exposure to dangerously high temperatures, and desiccation. Typical inhabitants of the intertidal rocky shore include urchins, sea anemones, barnacles, chitons, crabs, isopods, mussels, starfish, and many marine gastropod molluscs such as limpets and whelks.

As with the dry sand part of a beach, legal and political disputes can arise over the ownership and use of the foreshore. One recent example is the New Zealand foreshore and seabed controversy. In legal discussions, the foreshore is often referred to as the wet-sand area.

For privately owned beaches in the United States, some states such as Massachusetts use the low-water mark as the dividing line between the property of the State and that of the beach owner; however the public still has fishing, fowling, and navigation rights to the zone between low and high water.[4] Other states such as California use the high-water mark.

In the United Kingdom, the foreshore is generally deemed to be owned by the Crown although there are notable exceptions, especially what are termed several fisheries, which can be historic deeds to title, dating back to King John's time or earlier, and the Udal Law, which applies generally in Orkney and Shetland.

In Greece, according to the L. 2971/01, the foreshore zone is defined as the area of the coast that might be reached by the maximum climbing of the waves on the coast (maximum wave run-up on the coast) in their maximum capacity (maximum referring to the "usually maximum winter waves" and of course not to exceptional cases, such as tsunamis etc.). The foreshore zone, a part of the exceptions of the law, is public, and permanent constructions are not allowed on it.

The Intertidal Zone was used as a title for Stephen Hillenburg's old comic strip. The comic strip starred "Bob The Sponge", who became SpongeBob SquarePants.

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  Mussels in the intertidal zone in Cornwall, England.
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  Barnacles and limpets in the intertidal zone near Newquay, Cornwall, England.
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  A tidal pool in the intertidal zone during low tide, Sunrise-on-Sea, South Africa.
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  Unexplained crumbs of sand that appear to have been deposited around stone by escaping air.
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  Rocks in intertidal zone completely covered by mussels, at Bangchuidao Scenic Area, Dalian, Liaoning Province, China.

• Ballantine Scale
• Ecological forecasting
• Littoral series
• NaGISA
• Shorezone

Wikisource has the text of the 1911 Encyclopædia Britannica article Foreshore.

• Enchanted Learning
• Encyclopædia Britannica
• Watch the online documentary The Intertidal Zone