Bacterial metabolism is a central driver of organic carbon transformation in the ocean, yet its coupling to distinct biogenic carbon pools produced by phytoplankton remains poorly constrained. We conducted seven cruises across diverse environmental gradients in the Taiwan Strait, a subtropical marginal sea of the Pacific, simultaneously quantifying bacterial metabolic rates alongside phytoplankton-derived particulate primary production (PPP) and dissolved primary production (DPP) to examine their relationships with bacterial metabolism. Phytoplankton-released DPP was generally insufficient to meet bacterial carbon demand, indicating substantial reliance on dissolved organic carbon regenerated from particulate organic matter through food-web processing. Bacterial production showed a stronger association with DPP, whereas bacterial respiration was more closely associated with PPP. Accordingly, bacterial growth efficiency (BGE) co-varied with the fraction of primary production released in dissolved form, indicating systematic associations between phytoplankton carbon partitioning and bacterial metabolic balance. Bacteria-phytoplankton coupling further varied across distinct physical water masses. River-plume waters exhibited lower BGE and lower DPP fractions, whereas offshore oligotrophic waters showed higher BGE and higher DPP fractions. Upwelling and mixed waters displayed similar DPP fractions but differed in BGE and bacterial carbon demand, reflecting variability in environmental conditions and carbon supply dynamics. These findings provide observational constraints on bacteria-phytoplankton coupling and microbial carbon flow pathways in dynamic marginal seas.

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