One Dimensional Ecosystem Model Simulation of the Effects of Vertical Dilution by the Winter Mixing on the Spring Diatom Bloom |
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| Authors: | Naoki Yoshie Yasuhiro Yamanaka Michio J Kishi Hiroaki Saito |
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| Institution: | (1) Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan;(2) Frontier Research System for Global Change, Global Warming Research Program, Showamachi, Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan;(3) Faculty of Fisheries, Hokkaido University, Minato-cho, Hakodate 041-8611, Japan;(4) Tohoku National Institute of Fisheries, Niihama-cho, Shiogama 985-0001, Japan |
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| Abstract: | A one-dimensional ecosystem model has been used to investigate the processes relevant to the spring diatom bloom which play
important roles in the biogeochemical cycle in the western subarctic Pacific. The model represents the plankton dynamics and
the nutrient cycles in the spring diatom bloom; its results show the importance of dilution by deep mixing in winter. It is
supposed that the vertically integrated biomass of phytoplankton decreases in the winter due to the decrease of photosynthesis,
because the deep mixing transports phytoplankton to a layer with a low light level. However, the observed integrated diatom
biomass increases as the mixed layer deepens. This is because the decrease of concentration due to dilution by mixing causes
the diatom grazed pressure to be less significant than diatom photosynthesis. In other words, the effect of dilution on the
grazed rate is more significant than the effect on the photosynthesis rate because the grazed rate depends on the concentrations
of both diatom and grazer, whereas the photosynthesis rate depends only diatom concentration. The average specific diatom
grazed rate, defined as grazed rate divided by diatom biomass, decreases by 35% associated with the deepening, while the average
specific photosynthesis rate of diatom decreases by 11%. As a result, the average specific net diatom growth rate during the
deep mixing is about 70% of its maximum during the spring diatom bloom. The deep mixing significantly affects the amplitude
of the spring diatom bloom not only by the supply of nutrients but also by the dilution which drastically decreases the grazed
pressure.
This revised version was published online in July 2006 with corrections to the Cover Date. |
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| Keywords: | Ecosystem model diatom spring bloom photosynthesis rate grazing pressure western North Pacific |
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