The influence of turbulent mixing on the subsurface chlorophyll maximum layer in the northern South China Sea

• Published in: Journal of oceanology and limnology Vol. 39; no. 6; pp. 2167 - 2180
• Main Authors: Shang, Chenjing, Liang, Changrong, Chen, Guiying, Gao, Yongli
• Format: Journal Article
• Language: English
• Published: Heidelberg Science Press 01.11.2021; Springer Nature B.V; Shenzhen Key Laboratory of Marine Bioresources and Eco-environmental Science,College of Life Science and Oceanography,Shenzhen University,Shenzhen 518060,China%State Key Laboratory of Tropical Oceanography,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 510301,China; Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou),Guangzhou 511458,China%Equipment Public Service Center,South China Sea Institute of Oceanology,Chinese Academy of Sciences,Guangzhou 510301,China
• Subjects: Chlorophyll; Chlorophyll a; Earth and Environmental Science; Earth Sciences; Fluxes; Internal waves; Microstructure; Nutrients; Oceanography; Phosphates; Physics; Phytoplankton; Turbulent mixing; Water circulation; Water column; South China Sea; chlorophyll; diapycnal diffusivity; turbulent mixing; nutrients; nutrient flux; chlorophyll a
• ISSN: 2096-5508; 2523-3521
• DOI: 10.1007/s00343-020-0313-1

We present observations from deployments of turbulent microstructure instrument and CTD package in the northern South China Sea from April to May 2010. From them we determined the turbulent mixing (dissipation rate ε and diapycnal diffusivity κ ), nutrients (phosphate, nitrate, and nitrite), nutrient fluxes, and chlorophyll a in two transects (A and B). Transect A was located in the region where turbulent mixing in the upper 100 m was weak ( κ ∼10 −6 −10 −4 m 2 /s). Transect B was located in the region where the turbulent mixing in the upper 100 m was strong ( κ ∼10 −5 −10 −3 m 2 /s) due to the influence of internal waves originating from the Luzon Strait and water intrusion from the western Pacific. In both transects, there was a thin subsurface chlorophyll maximum layer (SCML) (>0.25 mg/m 3 ) nested in the upper 100 m. The observations indicate that the effects of turbulent mixing on the distributions of nutrients and chlorophyll a were different in the two transects. In the transect A with weak turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients to the SCML but not to the upper water. Nutrients were sufficient to support a local SCML phytoplankton population and the SCML remained compact. In the transect B with strong turbulent mixing, nutrient fluxes induced by turbulent mixing transported nutrients not only to the SCML but also to the upper water, which scatters the nutrients in the water column and diffuses the SCML.