Iron and silicic acid concentrations together regulate Si uptake in the equatorial Pacific Ocean

The effects of added Si and Fe on the rate of silicic acid uptake were examined during two cruises to the equatorial Pacific upwelling zone between 110uW and 140uW. Maximum uptake rates of Si (Vmax) were highly consistent with a mean of 0.026 6 0.007 h21(n 5 29), implying maximum diatom growth rates of ,0.6 d21. Halfsaturation constants for Si uptake (KS) also showed little variance, averaging 1.7 6 0.7 mmol L21 Si(OH)4. No ecologically significant spatial or temporal patterns for either Vmax or KS were observed. Comparison of Si uptake rates at the ambient silicic acid concentration (Vamb) with Vmax indicated that the ambient [Si(OH)4] restricted Vamb to an average of 63% 6 13% of Vmax. Fe additions also caused significant increases in both Vmax and Vamb, indicating that the rate of Si uptake was also regulated by the ambient [Fe]. Fe additions had a variable effect on KS, but they consistently increased both Vmax and the initial slope of the kinetic curve (Vmax : KS), and thus the diatom assemblages’ ability to take up Si(OH)4 at low concentrations. Added Fe or Si increased Si uptake rates by 87% 6 59% and 69% 6 31%, respectively, indicating nearly equal roles for the two elements in limiting rates of Si uptake in situ. The largest average increase in Si uptake rates, 172% 6 43%, was observed when both Si and Fe were added, implying that together Si and Fe restricted Si uptake rates by almost a factor of three. The equatorial Pacific upwelling system is a major highnutrient low-chlorophyll (HNLC) region where relatively low phytoplankton productivity in nutrient-rich upwelled water is responsible for a large efflux of CO2 to the atmosphere (Landry et al. 1997; Takahashi et al. 2002). The mechanisms responsible for the HNLC condition of this region are still debated. In situ fertilization experiments (IRONEX I and II) have demonstrated that Fe limits phytoplankton biomass, nitrate use, and CO2 consumption in the equatorial Pacific (Martin et al. 1994; Coale et al. 1996b). However, low concentration of silicic acid, Si(OH)4, relative to nitrate, NO { 3 , in the waters upwelling along the equator have led to the hypothesis that new and export production in the equatorial Pacific are limited by Si 1 Present address: National Oceanography Centre, University of Southampton, European Way, Southampton S014 3ZH, United Kingdom. 2 Present address: Institut Universitaire Européen de la Mer, Technopole Brest-Iroise, Place Nicolas Copernic, Plouzané 29280, France.