Atmospheric deposition is recognized as an important source of nutrients in the upper ocean, and the nutrients it brings, such as nitrogen (N) and phosphorus (P), can affect the growth of marine phytoplankton and influence marine N cycle. The oligotrophic South China Sea (SCS) is particularly vulnerable to high anthropogenic nitrogen deposition due to the substantial populations and rapid economic development of neighboring countries. However, little is known about the mechanism of how anthropogenic activities influence the SCS system by interacting with the temporal shifts of nitrogen deposition. It is thus that the main challenges of atmospheric deposition to the ocean are discussed in this research from the aspects of bioavailable N and P as well as response of marine primary production process. Microcosm experiments in the SCS were conducted to quantify the effects of atmospheric nitrogen deposition on phytoplankton growth and microbiome community changes in August 2024. Four microcosms carried out with different ammonium-nitrate ratios (4.8, 1.5, 1.0 and 0.5) that simulate the evolution of deposition flux from 1980 to 2030 in general increased the nutrient concentration and promoted the growth of phytoplankton. Particularly the microcosm group that predict future deposition transition showed 2.6-fold and 3.7-fold increases in maximum Chl a and cell abundance of large-sized phytoplankton (> 5 μm), relative to the controls. Hence, our results suggested that the shift of nitrogen deposition pattern in southern China have stimulated different mechanisms of biologically available nitrogen in SCS, with major effects on the nutrient quality of coastal waters and species composition of marine phytoplankton communities.
 

Nitrogen deposition,South China Sea,Microcosm,Dynamic Response