Analysis of Long-Term Phytoplankton Dynamics with Environmental Factors in Harima-Nada, Eastern Seto Inland Sea, Japan

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  • 播磨灘における海洋環境と植物プランクトンの長期変動解析
  • ハリマナダ ニ オケル カイヨウ カンキョウ ト ショクブツ プランクトン ノ チョウキ ヘンドウ カイセキ

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Long-term monthly monitoring of phytoplankton with environmental factors was conducted at19sampling stations in Harima-Nada, eastern Seto Inland Sea, Japan for 36 years from 1973 to 2008. The yearly phytoplankton cell density and dissolvedinorganic nitrogen（DIN）concentration were both higher during the1970s to the early1980s, and then significantly decreased in the mid-1980s. Diatoms were the dominant phytoplankton group over the study period, and there was a marked shift from Skeletonema dominance to other diatoms, such as Chaetoceros, in the mid-1980s. High nutrient levels during the period from the 1970s to the early 1980s led to the large-scale red tides of the fish-killing raphidophyte Chattonella（Chattonella antiqua, Chattonella marina and Chattonella ovata）in Harima-Nada. However, nutrient levels continued to decrease thereafter, and Chattonella is considered to be unable to form large-scale blooms under the present conditions. The timing of germination of Chattonella cysts has become earlier due to the increase in water temperature, and the chances in vegetative growth have also increased, especially at the northern coast where most large rivers discharge into the Harima-Nada. During the 36-year period, Eucampia zodiacus cells were detected every year, and their abundance increased in the mid-1990s. Since the mid-1990s, blooms of E. zodiacus have been frequently detected during the nori harvest season and this species has become a predominant harmful organism indirectly causing the bleaching of aquacultured nori in Harima-Nada. There were two significant long-term changes, the increase in water temperature and decrease in the DIN concentration in winter. This shift is more advantageous for the growth of E. zodiacus, which has led to the domination of E. zodiacus in recent years.

Journal

  • Bulletin on Coastal Oceanography

    Bulletin on Coastal Oceanography 56 (2), 73-78, 2019

    Coastal Oceanography Research Committee, the Oceanographic Society of Japan

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