Mesoscale Eddies Drive Phytoplankton-Mediated Biogeochemistry in the South China Sea

Abstract

Ocean mesoscale eddies are important drivers of upper ocean physical and biological processes. However, owing to their ephemeral nature and limited observational data, the impact of eddies on three-dimensional biogeochemical cycles and hence related phytoplankton phenology remains unclear. Here, from ship-based surveys, we assessed the impact of two eddies of opposite polarity on phytoplankton biomass and community structure, in the upper 200 m of the northwest South China Sea (SCS), as well as their effect on the diapycnal nutrient fluxes and oxygen concentration. These observations revealed that pico-phytoplankton dominated phytoplankton community, whereas the fraction of micro- and nano- phytoplankton (F_micro and F_nano) increased with depth, reaching a maximum near the SCM layer (located between 50 and 100 m). The magnitude of SCM and total phytoplankton Chl were greater within the cyclonic eddy (CE) compared to those influenced by the anticyclonic eddy due to the enhanced vertical diapycnal fluxes of nutrients within the CE. The elevated diapycnal nutrient flux in the CE resulted from an increase in turbulent kinetic energy dissipation coefficient and steeper vertical gradients in inorganic nutrients. Pigment-based chemotaxonomy further indicated that eukaryotes increased significantly in the SCM layer with concentrations reaching 0.16 ± 0.08 mg m⁻³; the enhancement of F_micro in the CE was mainly attributed to the increased contribution of diatoms. The vertical biogeochemical dynamics revealed by this research may showcase fundamental characteristics of oligotrophic ecosystems, where mesoscale perturbations are vertically heterogeneous, improving our understanding of the complex biophysical interactions within mesoscale eddies.