Differential degradation pathways of organic matter driven by coastal fronts: Implications for carbon burial efficiency in the Beibu Gulf.

Coastal fronts create critical transition zones where intensified physical-biological coupling regulates aquatic biogeochemical cycles and ecosystem services. However, mechanistic insights of organic matter (OM) dynamics in response to frontal processes, along with their associated carbon sequestration and potential eco-environmental effects, remain poorly constrained due to a paucity of field observations. This study conducted a systematic investigation of dissolved OM (DOM) characteristics across seasonal cruises, integrating satellite remote sensing data, particulate OM (POM) and physicochemical parameters profiling to elucidate frontal controls on OM dynamics. Our findings demonstrated front-dependent modulation of OM processing pathways. In the high-intensity northern frontal zone, enhanced vertical mixing supplied abundant nutrients, stimulating phytoplankton growth. The dominant OM processing pathway therefore involves the phytoplankton production and subsequent degradation of fresh POM, releasing abundant labile DOM into the water column. By contrast, in the low-intensity eastern frontal zone, despite substantial DOM input via the West-Guangdong Coastal Current (with >40% of the annual input occurring in winter), weak frontal intensity failed to supply adequate nutrients to sustain autochthonous production, shifting the system toward direct microbial degradation of DOM as the primary removal process. These findings demonstrate that frontal activity serves as a hotspot for the dynamic processing of both POM and DOM in the water column, providing insights into how frontal dynamics influence coastal carbon cycling and highlighting the potential vulnerability of carbon sinks under future climate change.