Coastal systems are key components of the ocean carbon cycle and can substantially influence regional carbon sequestration and air-sea CO₂ exchange. Within these environments, coastal macroalgal aquaculture has been proposed as a nature-based carbon-removal pathway, yet the net CO₂ source-sink behavior of intensively farmed coastal waters remains poorly resolved. Here, we present a three-year high-resolution time series (2023-2025) of surface water CO₂ partial pressure (pCO₂) and concurrent environmental measurements from an autonomous buoy deployed in Dongwuyang, a semi-enclosed bay in southeastern China, to investigate surface water pCO₂ variability and its major controls across diel, seasonal, and interannual timescales. Surface water pCO₂ exhibited pronounced temporal variability, with a three-year average of 723 μatm, substantially higher than the atmospheric pCO₂ of 428 μatm. At the diel scale, pCO₂ oscillations closely followed tidal cycles, with high pCO₂ at high tide and low pCO₂ at low tide. Relative to the NpCO₂ (temperature-normalized pCO₂) baseline before the beginning of kelp culture, negative non-temperature anomalies during periods of active kelp growth was attributed to photosynthetic pCO₂ drawdown. However, these drawdown signals were intermittent and insufficient to offset the overall temperature effect (high pCO₂ at high temperature) and mixing of high pCO₂ waters. Dongwuyang therefore functioned predominantly as a net CO₂ source, with a mean air-sea CO₂ flux of 11.72 mmol m^-2 d^-1. This source behavior contrasts with the net atmospheric CO₂ uptake reported for some temperate macroalgal aquaculture systems, highlighting the strong site dependence of CO₂ source–sink behavior in macroalgal aquaculture systems. These findings demonstrate that high macroalgal productivity alone does not ensure net atmospheric CO₂ removal and that initial pCO₂ level, hydrodynamic exchange, and whole-system air-sea CO₂ fluxes must be considered when evaluating the climate-mitigation potential of macroalgae aquaculture.