Increased kinetic efficiency for silicic acid uptake as a driver of summer diatom blooms in the North Pacific subtropical gyre

Abstract

We examined Si limitation of silica production as a factor contributing to summer‐bloom dynamics in the North Pacific subtropical gyre (NPSG). Substrate limitation of Si uptake was pervasive, but in most samples the degree of Si limitation was not severe enough to induce growth limitation as observed in the North Atlantic subtropical gyre. These results revise previous interpretations, because the difference in the degree of kinetic limitation between gyres appears to be driven by differences in both substrate and kinetic efficiency. In the NPSG, diatom bloom assemblages tended to show more efficient Si‐uptake kinetics and higher Si‐uptake rates than did nonbloom assemblages. A bloom dominated by Mastogloia woodiana exhibited the most efficient Si‐uptake kinetics observed in the ocean to date. Enhanced kinetic efficiency for Si uptake was previously hypothesized to facilitate bloom development, but we suggest the efficient kinetics indirectly affects blooms because [Si(OH)4] does not appear to limit growth or division rates. Rather, efficient kinetics allows further depletion of the prevailing low [Si(OH)4] without inducing secondary growth limitation by Si, thereby maximizing biomass yield. Si‐uptake limitation was not detected in the lower euphotic zone despite similar [Si(OH)4] as in well‐lit waters, which suggests transition to light limitation of Si uptake with depth. These results suggest that enhanced silica production in the M. woodiana bloom was driven by a synergy between diatom species composition, where M. woodiana was highly efficient at Si uptake, and high irradiance, which lead to enhanced diatom division rates in the upper euphotic zone.