Seasonal changes of the vertical distribution of living planktic foraminifera at the main axis of the Kuroshio off Honshu, Japan

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  • 本州沖黒潮流軸部に生息する現生浮遊性有孔虫の深度分布とその季節変化
  • ホンシュウオキ クロシオリュウジクブ ニ セイソク スル ゲンセイ フユウセイユウコウチュウ ノ シンド ブンプ ト ソノ キセツ ヘンカ

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Abstract

The seasonal changes of standing stocks, relative abundance and vertical distributions of living planktic foraminifera in the main axis of the Kuroshio off the southern coast of Honshu in Japan were investigated during the period from May 1993 to December 1995. Forty plankton tow samples (>330μm) were mainly collected from 8 different water depths between 0 and 800m in September, December, January, March and May. Total foraminiferal standing stocks are abundant in March and December. The seven dominant species are composed of Globigerinoides sacculifer in September, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata in December, Globorotalia hirsuta, Globorotalia truncatulinoides and Globorotalia inflata in March, and Globorotalia menardii in May on the basis of the foraminiferal standing stocks. One of the reasons for these seasonal changes is the change in water temperature and vertical structure of the water column during the development of seasonal thermocline or active vertical mixing. Four periods of formation, flourish, decline and extinction of the seasonal thermocline which are distinguished in this area by the development of the vertical profiles of temperature and the structure of water mass correspond to May, September, December and March, respectively. Based on the standing stocks of planktic foraminifera, the maximum concentration of Globigerinoides sacculifer occurs within and above the remarkable seasonal thermocline in September. The highest concentations of Neogloboquadrina dutertrei and Pulleniatina obliquiloculata in December and those of Globorotalia menardii in May are encountered within the seasonal thermocline. The maximum concentrations of Globorotalia inflata occur within the active mixing layer (upper 200m) in March, while those of Globorotalia hirsuta and Globorotalia truncatulinoides occur below the mixing layer. Although Globorotalia inflata and Globorotalia truncatulinoides were not included in the 25 species of the Equatorial west-central assemblage, relatively high standing stocks of these two species are encountered at the present studied area during the period from December to March.

Journal

  • Fossils

    Fossils 70 (0), 1-17, 2001

    Palaeontological Society of Japan

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