Seasonal variations in the abundance and sinking flux of biogenic silica in Daya Bay, northern South China Sea

Summary: Coastal seas account for >50% of the biogenic silica (BSi) production in marine environments. However, BSi sinking is poorly understood. Here, seasonal variations in the abundance and sinking flux of BSi were investigated in Daya Bay, in the northern South China Sea. The highest BSi concentrations occurred in summer, averaging 8.04 ± 5.48 μmol L−1 (±SD), followed by autumn (5.51 ± 3.11 μmol L−1) and spring (3.76 ± 3.06 μmol L−1). The lowest BSi concentration of 2.93 ± 1.34 μmol L−1 was observed in winter. Based on 234Th/238U disequilibria, the average BSi sinking fluxes were 7.08 ± 8.62, 10.01 ± 13.95, and 8.30 ± 13.06 mmol Si m−2 d−1 in spring, summer, and autumn, respectively. The lowest flux of 4.19 ± 3.98 mmol Si m−2 d−1 was measured in winter. Together with nitrogen fluxes, the Si:N sinking ratios were 0.8:1.0, 1.5:1.0, 1.6:1.0, and 1.4:1.0 in spring, summer, autumn, and winter, respectively, indicating that particle sinking induces the faster removal of Si compared to N in Daya Bay. Keywords: Biogenic silica, Dissolved silicate, Sinking flux, Thorium, Daya Ba