Aquatic plant distribution of irrigation ponds in relation to land use and water quality in an agricultural landscape dominated by citrus orchards in Hojo area, Shikoku Island, southwestern Japan

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  • 愛媛県旧北条市の柑橘園地域におけるため池の水草の分布と土地利用・水質との関係

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Abstract

Rice fields are irrigated throughout the season from planting to harvesting. Irrigation ponds are constructed in areas where people can not get enough water from rivers. Particularly, there are numerous ponds in the Seto Inland Sea district of western Japan. As a case study of irrigation ponds, we studied water quality and aquatic plants of ponds in Hojo area, Shikoku Island, in the Seto Inland Sea district. The area's landscape is characterized by citrus orchards distributed widely on hillside slopes. Many ponds have watersheds partly or completely occupied by orchards. Ponds located in agriculturally intensive watersheds had extremely eutrophic water. Ponds located in forested watersheds had lower nutrient levels than the ponds in agricultural lands. Submerged and free-floating taxa dominated the aquatic flora. Many of them are known wet rice field weeds. Floating-leaved taxa were rare except Trapa japonica. Very eutrophic ponds were populated only with Lemnaceae species. Species-rich flora were restricted to the nutrient-poor and clear ponds located in forested watersheds. Repair work on two ponds resulted in recovery of floral diversity under a decline in phosphorus concentration. This suggests that land use changes, including citrus farming expansion, caused sensitive species to disappear. It also suggests the possibility of restoration and conservation of species-rich aquatic flora by environmental improvement where seed banks contain formerly diverse flora. Species composition differs even between ponds of a similar environment within a small geographical area. Therefore, it is desirable to conserve diverse ponds as much as possible to conserve diverse aquatic flora.

Journal

  • Vegetation Science

    Vegetation Science 26 (2), 65-78, 2009

    The Society of Vegetation Science

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