Macroalgal metabolism and lateral carbon flows create extended atmospheric CO₂ sinks

<jats:p>Abstract. Macroalgal beds have drawn attention as one of the vegetated coastal ecosystems that act as atmospheric CO2 sinks. Although macroalgal metabolism as well as inorganic and organic carbon flows are important pathways for CO2 sequestration by macroalgal beds, the relationships between macroalgal metabolism and associated carbon flows are still poorly understood. In the present study, we investigated carbon flows, including air–water CO2 exchange and budgets of dissolved inorganic carbon, total alkalinity, and dissolved organic carbon (DOC) in a temperate macroalgal bed during productive months of the year. To assess the key mechanisms of CO2 sequestration by the macroalgal bed, we estimated macroalgal metabolism and lateral carbon flows using a field-bag method, a degradation experiment, and mass balance modelling over a diurnal cycle. Our results showed that macroalgal metabolism and lateral carbon flows driven by water exchange affected air–water CO2 exchange in the macroalgal bed and the surrounding waters. Macroalgal metabolism caused overlying waters to contain low concentrations of CO2 and high concentrations of DOC that were efficiently exported offshore from the macroalgal bed. The exported water lowered CO2 concentrations in the offsite surface water and enhanced atmospheric CO2 uptake. Our findings suggest that macroalgal beds in habitats associated with high water exchange rates can create extensive CO2-sinks around them. </jats:p>