Abstract
The response of silicic acid uptake by phytoplankton populations to varyingsilicic acid concentrations has been investigated using a newly developed Si stable isotope tracer technique in the Peru upwelling region. Since silicic acid uptake follows the Michaelis-Menten expression for enzyme kinetics, a hyperbola describes the relationship between concentration and uptake. The half saturation constant ( K τ ) for uptake and the maximum uptake ( V max) were 2·93 μg atoms Si. 1. −1 and 0·075 hr −1, respectively. The uptake is light-dependent and this relation is also described by the Michaelis-Menten equation. Since uptake is both light- and silicic acid concentration-dependent, vertical profiles of it obtained with the Si isotope tracer technique can be interpreted from the interaction of light and silicic acid concentrations at different depths. The uptake of silicic acid by natural phytoplankton populations had diel periodicity, with a maximum about noon and significant nearly uniform rate at night. Nitrate reductase activity and 15N-nitrate and 15N-ammonium uptake had a similar diel periodicity.
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