Spatial Variability in Microzooplankton Grazing and Its Relationship to Carbon Export Efficiency in the South China Sea During the Summer

Microzooplankton grazing is a key regulator of phytoplankton standing stock and the export of particulate organic carbon (POC) in the ocean, yet its spatial coupling with phytoplankton growth across shelf-basin gradients remains underexplored in marginal seas. Using 43 dilution experiments in the South China Sea (SCS), this study systematically examined the spatial variability and environmental drivers of phytoplankton growth rate (μ) and grazing mortality rate (m) as well as the resulting grazing impact (m/μ) across shelf, slope, and basin habitats. Results show a significant seaward decline in m (shelf: 0.73 ± 0.53 d⁻¹; basin: 0.27 ± 0.19 d⁻¹; p < 0.01), while μ decreased non significantly (shelf: 0.88 ± 0.53 d⁻¹; basin: 0.70 ± 0.18 d⁻¹; p = 0.49). Consequently, growth and grazing were strongly coupled on the shelf (r = 0.82, p < 0.01) but completely decoupled in the basin (r = 0.21, p = 0.46), coinciding with a shift toward picophytoplankton dominance and reduced grazing pressure. We also found that export efficiency was positively correlated with both m/μ and m, highlighting the important role of microzooplankton grazing in carbon export, but that the overall carbon export flux itself was also shaped by nutrient-driven changes in POC recycling. These findings underscore the role of bottom-up nutrient forcing and top-down grazing impact in controlling carbon export in the SCS.