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.
| Original language | English |
|---|---|
| Article number | e2024JG008664 |
| Number of pages | 19 |
| Journal | Journal of Geophysical Research: Biogeosciences |
| Volume | 130 |
| Issue number | 6 |
| Early online date | 6 Jun 2025 |
| DOIs | |
| Publication status | Published - 6 Jun 2025 |
Funding
This work was funded by the National Natural Science Foundation of China (Grants 42406187, 42376117, and 42276189) and the Natural Science Foundation of Jiangsu Province (Grant BK 20241027). M.C. was supported by the National Science Foundation (award: OCE 2023274/2147809 to A.J. Fassbender), National Oceanic and Atmospheric Administration (NOAA), Global Ocean Monitoring and Observing program, and NOAA Pacific Marine Environmental Laboratory. AGH was supported by the Danish National Centre for Climate research (NCKF). We are also grateful to Wen Zhou, Yuan Dong, and Xiangfu Li (South China Sea Institute of Oceanology, Chinese Academy of Sciences) for the HPLC analysis and nutrients analysis, respectively. Data and samples were collected onboard of R/V Shiyan VI implementing the open research cruise NORC2022-302 supported by NSFC Shiptime Sharing Project (project number: 42149907). We also extend our gratitude to the officers and crew of R/V Shiyan VI for their cooperation during the field cruise. The authors are grateful to everyone who worked hard collecting and analyzing the in situ data.
Keywords
- mesoscale eddies
- phytoplankton
- phenology
