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 110°W and 140°;W. Maximum uptake rates of Si ( V max ) were highly consistent with a mean of 0.026 ± 0.007 h −1 (n = 29), implying maximum diatom growth rates of ~0.6 d −1 . Half‐saturation constants for Si uptake ( K S ) also showed little variance, averaging 1.7 ± 0.7 mmol L −1 Si(OH) 4 . No ecologically significant spatial or temporal patterns for either V max or K S were observed. Comparison of Si uptake rates at the ambient silicic acid concentration ( V amb ) with V max indicated that the ambient [Si(OH) 4 ] restricted V amb to an average of 63% ± 13% of V max . Fe additions also caused significant increases in both V max and V amb , indicating that the rate of Si uptake was also regulated by the ambient [Fe]. Fe additions had a variable effect on K S , but they consistently increased both V max and the initial slope of the kinetic curve ( V max : K S ), 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% ± 59% and 69% ± 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% ± 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.