Smith¹, S. Lan, Naoki Yoshie², Chisato Yoshikawa², Masahiko Fujii³, Yasuhiro Yamanaka^{1, 2} and Michio J. Kishi^{1, 4}

¹FRSGC, 3173-25 Showa-machi, Kanazawa-ku, Yokohama, 236-0001 Japan, Tel: 011-81-45-778-5581, (Fax -5707), E-mail: lanimal@jamstec.go.jp, ²Hokkaido Univ., Sapporo, Japan; ³JSPS, Tokyo & NIES, Tsukuba, Japan, and ⁴Hokkaido Univ., Hakodate, Japan

Modeling coupled biogeochemistry and ecosystem dynamics at four stations in the North Pacific: recent results from the FRSGC-HU Group

We have developed one-dimensional models based on the NEMURO model by PICES (the North Pacific Marine Science Organization) and applied them to four stations in the North Pacific.

A study simulating the dynamics of Si/N ratios for Stn. A7 (41°N, 145°E) in the western North Pacific found that three Si/N ratios (for uptake by phytoplankton, composition of sinking particles, and dissolved nutrients) varied because of differences between Si and N cycles in the ecosystem, even when using constant stoichiometry for diatoms. In summer observed surface nutrients are depleted at A7 but not at Stn. KNOT (44°N, 155°E). Using the same maximum photosynthetic rate for diatoms, simulated surface nutrient concentrations at KNOT were too low, especially in summer. This may be because iron limitation is more severe at KNOT than at A7. To consistently simulate the data for various nutrients, organic matter, and primary production at Stn. ALOHA, diazotrophs, bacteria, a microbial food web and a P-cycle were added. The simulations only approached consistency with all the data when accounting for differential remineralization of C, N and P in organic matter and high C:N ratios for uptake by phytoplankton. A model including the isotopes ¹⁴N and ¹⁵N was applied to the Sea of Okhotsk. To simulate seasonal variations in δ ¹⁵N of sinking particles, one must consider not only the isotopic fractionation for nitrate assimilation, but also those for ammonium assimilation and nitrification.