Deep-Sea Research II 53 (2006) 2297–2326 Modeling studies investigating the causes of preferential depletion of silicic acid relative to nitrate during SERIES, a mesoscale iron enrichment in the NE subarctic Pacific S. Takeda a,Ã , N. Yoshie b , P.W. Boyd c , Y. Yamanaka b a Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-8657, Japan b Graduate School of Environmental Earth Science, Hokkaido University, N10W5 Kita-Ku, Sapporo 060-0810, Japan c National Institute of Water and Atmospheric Research (NIWA), Center for Chemical and Physical Oceanography, Department of Chemistry, University of Otago, Dunedin, New Zealand Received 25 March 2006; accepted 11 May 2006 Available online 11 October 2006 Abstract Numerical modeling experiments were conducted to examine the reasons for observed changes in the silicic acid ([Si(OH) 4 ]) to nitrate ð½NO  3 Þ drawdown ratio after the onset of algal iron stress during SERIES. During phytoplankton blooms and immediately after them, cells encounter a range of iron stress (between iron-replete and iron-deplete) and therefore show a range of growth rates. For these reasons, the potential influence of phytoplankton growth rate, under conditions of algal iron stress, on silicic acid and nitrate depletion were investigated in numerical experiments by altering the timing of a shift in the [Si(OH) 4 ]:½NO  3  uptake ratio. These simulations suggested that the continued growth of iron- stressed phytoplankton at sub-maximum rates, with an elevated [Si(OH) 4 ]:½NO  3  uptake ratio, induced depletion of silicic acid in the surface water and resulted in simultaneous limitation of growth by both iron and silicic-acid supply. Therefore, bottom-up control played an important role in terminating the phytoplankton bloom in SERIES. In the model simulations, the enhancement of diatom silicification due to increased rates of biomass-normalized silicic-acid uptake, led to increases in the export flux of opal after the onset of algal iron-stress and, consequently, it stimulated the silica pump. The regulation of both the [Si(OH) 4 ]:½NO  3  uptake ratio and the growth rate of phytoplankton by iron supply are important factors that determine the relative consumption of silicic acid and nitrate upon iron stress, although the potential influence of a floristic shift in the diatom assemblage cannot be ruled out. These findings offer insights into the impact of iron fertilization, both artificial and natural, on the biogeochemical cycling of nutrients in high-nitrate, low- chlorophyll waters. r 2006 Elsevier Ltd. All rights reserved. Keywords: Iron enrichment; Phytoplankton nutrient limitation; Nitrate; Silicic acid; Iron; Growth rate ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.05.027 Ã Corresponding author. Tel.: +81 3 5841 5290; fax: +81 3 5841 5308. E-mail address: atakeda@mail.ecc.u-tokyo.ac.jp (S. Takeda).