Contribution of marine macrophytes to pCO2 and DOC variations in human-impacted coastal waters

Carbon cycles in coastal waters are highly sensitive to human activities and play important roles in global carbon budgets. CO₂ sink–source behavior is regulated by spatiotemporal variations in net biological productivity, but the contribution of macrophyte habitats including macroalgae aquaculture to atmospheric CO₂ removal has not been well quantified. We investigated the variations in the carbonate system and dissolved organic carbon (DOC) in human-impacted macrophyte habitats and analyzed the biogeochemical drivers for the variations of these processes. Cultivated macroalgal metabolism (photosynthesis, respiration, calcification, and DOC release) was quantified by in situ field-bag experiments. Cultivated macroalgae took up dissolved inorganic carbon (DIC) (16.2–439 mmol-C m⁻² day⁻¹) and released DOC (1.2–146 mmol-C m⁻² day⁻¹). We estimated that seagrass beds and macroalgae farming contributed 0.8 and 0.4 mmol-C m⁻² day⁻¹ of the in situ total CO₂ removal (5.7 and 6.7 mmol-C m⁻² day⁻¹, respectively) during their growing period in a semi-enclosed embayment but efficient water exchange (i.e., short residence time) in an open coastal area precluded detection of the contribution of macrophyte habitats to the CO₂ removal. Although hydrological processes, biological metabolism, and organic carbon storage processes would contribute to the net CO₂ sink–source behavior, our analyses distinguished the contribution of macrophytes from other factors. Our findings imply that macroalgae farming, in addition to restoring and creating macrophyte habitats, has potential for atmospheric CO₂ removal.