Algal Succession on Different Substrata Covering the Artificial Iron Reef atIkata in Shikoku, Japan

Abstract

Devastation on coasts worldwide has led to barren grounds (Isoyake, Japanese) resulting in the loss of the natural population of many marine organisms and seaweeds (Serisawa et al. 1998;Choi et al. 2000a). In such coastal area, useful fishery resources such as abalone, fish and seaweeds have been depleted remarkably. In the 1970s technical studies on the restoration of Isoyake area (barren ground) to rich seaweed beds were begun under the leadership of the Fisheries Agency in Japan. Several studies on the succession of seaweeds have been carried out on artificial seaweed beds in subtidal zone by placing artificial substrata on the sea bottom since 1980s (Hirata 1986; Serisawa and Ohno 1995a, b;Choi et al. 2000b). Most of these artificial seaweed beds have been constructed by the purpose of growing only larger seaweeds (Tsuda and Kami 1973). On the other hand, artificial iron reefs are commonly used to increase the fishery resources on the deeper bottom more than 20-30 m depth in many coastal areas where there is no seaweed community. The artificial iron reefs, which have placed on the sandy bottom at 5-10 m depths, have been supposed to form good nursery ground for juvenile. The fishes come together with good feed on the seaweeds. But there are not reports on the seaweed bed cover on the artificial iron reef yet. Various substrata such as stone, concrete, wood and steel have made the artificial algal beds cover on the artificial reef in this study. These structures also act as artificial substrata for marine algae. The algal community colonizes on the artificial reefs and serves as the primary food source for herbivores (Watanuki and Yamamoto 1990). Choi et al. (2000a) has reported on early stage of algal succession on artificial reefs. This study has surveyed on marine algae on the climax succession growing Sargassum and Ecklonia at the different substrata cover on the artificial iron reef during the period of two years, where were placed on the sandy bottom, and such special seaweed beds were also compared to the seaweed beds near the rocky shore. Succession of artificial seaweed communities in an artificial iron reef at Ikata, southern Japan was studied based on monthly or bimonthly observations from February 1999 to August 2000. Communities were associated with different substrata (40 cm x 60 cm: steel, concrete, wood and stone) as the cover on artificial iron reefs (4.5 m x 4.1 m x 2.5 m, 45.38 m 3 and 3.2 ton), which were placed on a sandy substratum at 8 m depth. Within one month diatoms dominated on all substrata with cover of approximately 100%. Enteromorpha intestinalis and Colpomenia sinuosa dominated on the reef within three months after the placement in the spring. Seaweed communities on the reef decreased during the summer. In the winter, the seaweeds on the reef recovered. Sargassum spp., Ecklonia kurome and Padina arborescens dominated on each substratum after one year. Seaweed communities on the artificial reef were similar to those on the rocky substratum around the artificial reef and also sim...

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