TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 60 (No. 2), pp. 227-241, 2004

An Ecosystem Model Coupled with Nitrogen-Silicon-Carbon Cycles Applied to Station A7 in the Northwestern Pacific

Yasuhiro Yamanaka1,2*, Naoki Yoshie2, Masahiko Fujii3,4, Maki N. Aita1 and Michio J. Kishi1,5

1Global Warming Research Program, Frontier Reserch System for Global Change, Showa-machi, Kanazawa-ku, Yokohama 236-0001, Japan
2Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan
3Japan Society for the Promotion of Science, Ichiban-cho, Chiyoda-ku, Tokyo 102-8471, Japan
4National Institute for Environmental Studies, Onogawa, Tsukuba, Ibaraki 305-0053, Japan
5Faculty of Fisheries, Hokkaido University, Minato-cho, Hakodate 041-8611, Japan

(Received 20 June 2002; in revised form 5 April 2003; accepted 13 May 2003)

Abstract: A model based on that of Kishi et al. (2001) has been extended to 15 compartments including silicon and carbon cycles. This model was applied to Station A7 off Hokkaido, Japan, in the Northwestern Pacific. The model successfully simulated the observations of: 1. a spring bloom of diatoms; 2. large seasonal variations of nitrate and silicate concentrations in the surface water; and 3. large inter-annual variations in chlorophyll-a. It also reproduced the observed features of the seasonal variations of carbon dioxide partial pressure (pCO2)—a peak in pCO2 in winter resulting from deep winter convection, a rapid decrease in pCO2 as a result of the spring bloom, and an almost constant pCO2 from summer through fall (when the effect of increasing temperature cancels the effect of biological production). A comparison of cases with and without silicate limitation shows that including silicate limitation in the model results in: 1. decreased production by diatoms during summer; and 2. a transition in the dominant phytoplankton species, from diatoms to other species that do not take up silicate. Both of these phenomena are observed at Station A7, and our results support the hypothesis that they are caused by silicate limitation of diatom growth.

*Corresponding author E-mail: galapen@ees.hokudai.ac.jp

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