Seasonal dynamics and export of biogenic silica in the upper water column of a large marginal sea, the northern South China Sea

Biogenic silica (BSi), primarily derived from marine diatoms, is a major component of the oceanic silicon cycle. It links with the global carbon cycle through the silicate pump via photosynthesis and the so-called ballast effect in many areas of the world's oceans, particularly in ocean margins. Here we examined the distribution and export of BSi based on a large data set collected from the upper water column of the northern South China Sea (NSCS), the largest marginal sea of the Pacific Ocean. Generally, BSi concentrations decreased from the shelf to the basin in all seasons. The highest value was observed in nearshore surface waters in summer, associated with a diatom bloom in the far-reaches of river plumes. Seasonally, BSi concentrations were overall higher in autumn and winter than in spring and summer, due to elevated diatom productivity supported by an increased vertical supply of nutrients in dry seasons. Patterns of BSi spatial and seasonal variability were well represented by those of the pigment fucoxanthin, demonstrating the dominant role of diatoms in controlling BSi dynamics in the upper NSCS. Mainly regulated by BSi standing stock, BSi export flux from the euphotic zone, determined using thorium-234/uranium-238 disequilibrium, was on average higher on the shelf than in the basin, and in the dry than wet season. Furthermore, the high BSi export helped to promote the export of particulate organic carbon (POC) on the shelf, while the POC standing stock was comparable between different regions of the NSCS. Thus, the silicate pump, driven by diatoms in the upper water column, functioned more efficiently under eutrophic conditions. A comparison with prior measurements of particle sinking fluxes using sediment traps showed that the correlation of BSi and POC increased and became more significant with increasing water depth. This provides direct evidence of the pronounced ballast effect of BSi in transferring POC into deep waters of the NSCS basin, even though the contribution of diatoms to POC accumulation in the euphotic zone was minor in this oligotrophic environment.