Nearshore cultivation of seaweed holds promise for modulating the seawater carbonate system, mitigating local acidification and enhancing carbon sequestration, yet the temporal dynamics of these effects remain poorly characterized. Here we conducted simultaneous monitoring of surface-seawater pH, dissolved oxygen (DO) and partial pressure of CO₂ (pCO₂) over a complete growth cycle of the Saccharina japonica in a cultivation area and an adjacent control site. The measurements resolved both semidiurnal tidal fluctuations and progressive changes across growth stages, allowing assessment of the evolving carbon source–sink function. Water-column parameters displayed clear semidiurnal tidal variability throughout the study. With kelp growth, pH and DO concentration in the cultivation area increased steadily to mid-to-late-stage weekly means of 8.14 ± 0.1 and 14.80 ± 0.23 mg L⁻¹, respectively, while surface pCO₂ fell to 289 ± 93 μatm. Cultivation-area pH and DO were significantly higher than control values. The local CO₂ source–sink status of the water column evolved sequentially from a source through near-equilibrium to a sink, before returning to near-equilibrium.These results show that Saccharina japonica cultivation can raise seawater pH and DO and enhance stage-dependent carbon-sink capacity.