Impacts of Mesoscale Eddies on Production and Transformation of Dissolved Organic Matter in the Central South China Sea

Abstract Marine mesoscale eddies significantly influence the vertical transport of dissolved organic matter (DOM), yet their role in DOM production and transformation in the oligotrophic open ocean remains poorly constrained. Here, we investigated DOM dynamics associated with an anticyclonic–cyclonic eddy (AE–CE) pair in the oligotrophic central South China Sea through analysis of high‐resolution profiles of dissolved organic carbon (DOC), optical DOM properties, and related biogeochemical parameters such as NO 3 − , chlorophyll‐ a (Chl‐ a ), Chl‐a‐derived pheopigments (Pheo‐a), and microbial community composition. Three fluorescent components were identified with parallel factor analysis: two humic‐like, and one protein‐like. The CE uplifted humic‐rich low‐DOC subsurface water, whereas the AE transported protein‐rich high‐DOC surface water downward. In the CE, net DOC production was coupled well with Chl‐a distribution, indicating DOM production promoted by enhanced primary production due to upwelled nutrients. Concurrently, the net consumption of protein‐like FDOM, production of humic‐like FDOM, and high abundance of heterotrophs indicated microbial transformation of labile DOM into refractory compounds. The AE exhibited higher net DOC production, primarily protein‐like. A higher ratio of Pheo‐a to Chl‐a in the inventory suggested that DOC production in the AE was likely derived from the release of dead phytoplankton cells, a process associated with the downwelling of oligotrophic surface waters. This study reveals that the open‐ocean eddy pairs reshape microbial community structure and directly modulate the production and transformation of DOM, thereby significantly influencing carbon cycling and food web dynamics in the oligotrophic ocean.