Intertidal zone

A part of the marine ecosystem located at the junction of land and sea between the highest and lowest tides of spring tides. This is a place where the tides repeatedly cause dry spells and floods, and where environmental conditions such as temperature, humidity, salinity, and light intensity change rapidly and dramatically, resulting in the formation of a unique biological community of organisms that can tolerate these conditions. The intertidal zone is divided into rocky reefs, boulders, sandy beaches, muddy beaches, etc., depending on the bottom material, and can also be divided into manmade structures such as cliffs, revetments, and bridge piers, and natural intertidal zones. Each of these zones creates a different environment for organisms depending on the degree of influence of river water. Due to these differences, intertidal biological communities can take on a variety of forms, from muddy tidal flats to rough rocks, but in all cases, a biological zonation structure that runs roughly parallel to the tide level can be seen from the high tide line to the low tide line. The width of this distribution zone changes significantly depending on wave conditions, and is not necessarily determined by the tidal range itself. For this reason, the biological community itself is often used as a guide for classification. For example, on rocky coasts, the area from the upper limit of growth of large brown algae and coral to the upper limit of the barnacle zone is called the true intertidal zone, the area above that to the upper limit of the periwinkle zone is called the upper edge of the intertidal zone, and the area from the upper limit of the large brown algae to the lowest spring tide line is called the upper edge of the subintertidal zone. The stronger the waves, the higher and wider the upper limits of each zone are. On rocky reefs, sessile organisms dominate, with many filter feeders such as barnacles and mussels, grazers such as periwinkles, shells and limpets, and predators such as lycanthropes. It is thought that the upper limit of distribution of each organism is determined by abiotic factors such as dryness due to drying out and temperature, while the lower limit is determined by biotic factors such as predation and interspecific competition.

[Moritake Nishihira]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

大潮の最満潮線と最干潮線間の海と陸の接点に位置する海洋生態系の一部。潮の干満により干出冠水を繰り返す場所で、温度、湿度、塩分、光量などの環境条件の変化が急激で大きく、それに耐性を有する生物からなる独自の生物群集が成立する。潮間帯はその底質により岩礁、転石、砂浜、泥浜などに区分され、岸壁、護岸、橋脚などの人工物と自然の潮間帯に分けることもできる。それぞれがさらに河川水の影響の大小により生物にとって異なる環境になる。潮間帯生物群集はこれらの差異により泥底干潟から荒磯(あらいそ)までさまざまな様相を呈するが、いずれの場合にも高潮線から低潮線にかけてほぼ潮位と平行する生物の成帯構造がみられる。この分布帯の幅は波浪条件によって著しく変化し、かならずしも潮差そのものによって決まらない。そのために生物群集自身を目安に区分することが多い。たとえば、岩礁海岸では大形褐藻やサンゴなどの生育上限からフジツボ帯の上限までを真潮間帯、その上部のタマキビ帯上限までを潮間帯上縁部、大形褐藻類の上限から大潮最干潮線までを亜潮間帯上縁部という。それぞれの上限は波浪が強いほど高くまた幅も広い。岩礁では固着性生物が優占し、フジツボやイガイなどの濾過(ろか)食者、タマキビ、アマガイ、カサガイなどのグレイザー、レイシガイ類の捕食者が多い。各生物の分布上限は干出に伴う乾燥や温度などの非生物的要因により、下限は被食や種間競争などの生物的要因によって決まると考えられている。

［西平守孝］

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