Evaluation of Silicic Acid Sources for Spring Diatom Blooms on the Continental Shelf: Insights FromStable Silicon Isotopes in the East China Sea

The concentration of dissolved silicic acid (DSi) and its isotopic composition (δ³⁰Si(OH)₄) were determined for the East China Sea (ECS) shelf in the spring season with high spatial resolution. DSi concentrations and δ³⁰Si(OH)₄ values ranged from 1.3 to 116.5 μM and +0.85‰ to +2.53‰, respectively. An enrichment of ³⁰Si was observed in most surface-water samples compared with bottom waters. Several shallow-water stations at water depths of <20 m reflected the overall control of biological utilization of DSi and regeneration of DSi from biogenic silica (BSi) on the distribution of δ³⁰Si(OH)₄ values. A seasonal difference in δ³⁰Si(OH)₄ values was observed in the euphotic zone of shelf waters, which is attributed mainly to continuous consumption of DSi in enhanced diatom blooms. The major water masses influencing the ECS shelf were characterized with distinct DSi concentrations and isotopic compositions. Using Si isotopic constraints, modeling of δ³⁰Si(OH)₄ values implied that utilization of DSi in the ECS occurred as if in a steady-state mode. Riverine input was the major DSi source for the surface waters of inner-shelf areas, as well as part of the middle- and outer-shelf surface waters near the Changjiang River estuary. Upwelled Kuroshio Subsurface Water from the shelf break supported diatom growth in middle-shelf surface waters. Taiwan Strait Warm Water and Kuroshio Surface Water were relatively oligotrophic, and their influence was confined to waters near the Taiwan Strait and the outer shelf and slope areas.