Photosynthetic organisms shift the dynamics of surface pCO₂ driven by the sea surface temperature change (thermodynamic driver) by assimilating C from seawater. Here we measured net C uptake in a macroalgal habitat of coastal Korea for two years (2019–2020) and found that the macroalgal habitat contributed 5.8 g C m^-2 month^-1 of the net C uptake during the growing period (the cooling period, September-May). This massive C uptake changed the thermodynamics-driven seasonal dynamics such that the air-sea equilibrium of pCO₂ was pushed into disequilibrium. The surface pCO₂ dynamics during the cooling period were mostly influenced by the seasonal decrease in temperature and the proliferation of macroalgae, while the dynamics during the warming period (the stagnant period, June-August) closely followed that predicted based solely on the change in sea surface temperature only (thermodynamic driver). In contrast to the phytoplankton-dominated offshore waters (where phytoplankton populations are large in spring and summer), the impact of coastal water macroalgae on surface pCO₂ dynamics was most pronounced during the cooling period, when the magnitude of pCO₂ change was as much as twice that resulting from temperature change. Our study shows that the distinctive features of the macroalgal habitat—in particular the seasonal temperature extremes (~18°C difference), the active macroalgal metabolism, and anthropogenic nutrient inputs—collectively influenced the seasonal decoupling of seawater and air pCO₂ dynamics.

Continuous measurement

Surface-water pCO₂ was continuously measured from January to September in 2019 and from March to December in 2020. The mooring system was anchored to the bottom of the macroalgal habitat (10 m depth), and the pCO₂ sensor package was submerged approximately 0.5 m below the surface. Surface-water pCO₂ (or pH), temperature and salinity were concurrently measured at 1-h intervals.

Discrete measurement

Discrete surface samples for seawater C parameters (pH, total alkalinity, and total dissolved inorganic carbon) and nutrients were measured at 3–4 d intervals.