Seaweed bed

A seaweed bed is a colony of submerged seaweed (marine flowering plants) or large seaweed that grows in shallow areas along the ocean coast. Seaweed beds can be broadly divided into two types: eelgrass beds (or Zostera marina beds), which are colonies of the Zostera marina family that grow on calm coasts and in the sandy, muddy areas of inland bays, and Sargassum beds, which are made up of Sargassum species that grow on rocky areas along the open sea coast. There are also colonies of large brown algae such as Eisenia bicolor, Ecklonia cava, and Laminaria japonica, which are called underwater forests due to their shape. These seaweed beds are not only visually striking, but also occupy an important position in the coastal ecosystem because they actively photosynthesize, are major primary producers along the coast, and have a complex and rich biota ranging from microscopic animals to fish. Many fish gather in seaweed beds, but the reason for this has not been fully elucidated. In the sea areas where seaweed beds exist, the amount of dissolved oxygen and organic matter in the water increases, and a huge variety and amount of organisms, such as diatoms, small algae, small shrimps, amphipods, small crustaceans such as isopods, small snail shells, bryozoans, and hydrozoans, attach to the surface of the seaweed, providing an abundance of food for fish and making them valuable as feeding sites. The existence of seaweed beds also plays a role in protecting young fish from predators. While some fish, such as blenny eels and gobies, spend their entire lives in seaweed beds, there are many important fish species that spend the early stages of their life cycle in seaweed beds, such as black porgy, sea bass, rockfish, and greenlings, and these places need to be protected. However, since the shallow coastal areas where seaweed beds exist are also suitable for industrial sites, many areas have been reclaimed or eelgrass beds have disappeared due to water pollution. Furthermore, the underwater forests of Eisenia and Ecklonia cava, which are formed by fish such as rabbitfish and sea urchins that eat herbivorous organisms, have been damaged, and many areas have become barren. For this reason, attempts are being made to artificially create seaweed beds.

[Kiyoshi Yoshiwara]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

海洋の沿岸の浅所に生育する沈水性の海草（海産顕花植物）や大形海藻の群落を藻場とよぶ。藻場には大別して、波の静かな沿岸や内湾の砂泥地に生育するヒルムシロ科のアマモの群落のアマモ場（またはアジモ場）と、外海沿岸の岩礁地帯に生育するホンダワラ類で形成されるガラモ場がある。そのほか、アラメ・カジメ類、コンブ類などの大形褐藻類の群落があるが、それらはその形状から海中林とよばれている。これらの藻場は、景観的に目だつばかりでなく、活発に光合成を行い、沿岸における有力な一次生産者であることや、微小動物から魚類に至る複雑豊富な生物相を有することで、沿岸生態系のなかで重要な位置を占めている。藻場には多くの魚類が集まるが、その理由は十分に解明されていない。藻場の存在している海域では、水中の溶存酸素量、有機物が増加し、珪藻(けいそう)類、小型藻類、小型エビ類、端脚(たんきゃく)類、等脚類などの小型甲殻類、小型巻き貝、コケムシ類、ヒドロ虫類など、おびただしい種類と量の生物が藻の表面に付着し、魚類にとっては食物が豊富で、摂食場所としての意義がある。また藻場の存在が稚魚を外敵から守る役割をも果たしている。ギンポやハゼ類の仲間のように藻場で終生を過ごす魚類もあるが、クロダイ、スズキ、メバル、アイナメなどのように、その生活史の初期段階を藻場で過ごす重要魚族も多く、これらの場所を保護する必要がある。しかし、藻場の存在する沿岸浅所は工場立地としても適しているため、多くの場所で埋立てが行われ、あるいは水質汚濁などによってアマモ場が消失している。さらに、アイゴなどの魚類やウニ類など藻食性生物によって海中林を形成しているアラメ・カジメが被害を受け、磯焼け状態になっている海域も多い。このため人工的に藻場を造成する試みがなされている。

［吉原喜好］

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