Legacy Effects of Late Macroalgal Blooms on Dissolved Inorganic Carbon Pool through Alkalinity Enhancement in Coastal Ocean

Taking the world's largest green tide caused by the macroalga Ulva prolifera in the South Yellow Sea as a natural case, it is studied here if macroalgae can perform inorganic carbon sequestration in the ocean. Massive macroalgae released large amounts of organic carbon, most of which were transformed by microorganisms into dissolved inorganic carbon (DIC). Nearshore field investigations showed that, along with seawater deoxygenation and acidification, both DIC and total alkalinity (TAlk) increased significantly (both >50%) in the areas covered by dense U. prolifera at the late-bloom stage. Offshore mapping cruises revealed that DIC and TAlk were relatively higher at the late-bloom stage than at the before-bloom stage. Laboratory cultivation of U. prolifera at the late-bloom stage further manifested a significant enhancement effect on DIC and TAlk in seawater. Sulfate reduction and/or denitrification likely dominated the production of TAlk. Notably, half of the generated DIC and almost all the TAlk could persist in seawater under varying conditions, from hypoxia to normoxia and from air-water CO2 disequilibrium to re-equilibrium. The enhancement of TAlk allowed more DIC to remain in the seawater rather than escape into the atmosphere, thus having the longterm legacy effect of increasing DIC pool in the ocean.

Automated summary

This study investigates the ability of macroalgae to sequester inorganic carbon in the ocean, using the largest green tide caused by Ulva prolifera as a case study. The research shows that massive macroalgae release organic carbon, which is transformed into dissolved inorganic carbon (DIC) by microorganisms. Field investigations reveal that DIC and total alkalinity (TAlk) increase significantly in areas covered by dense U. prolifera, and offshore mapping cruises confirm higher levels of DIC and TAlk during the bloom stage. Laboratory cultivation of U. prolifera further demonstrates the enhancement effect of DIC and TAlk in seawater. This research highlights the importance of macroalgae in regulating the carbon pool in the ocean.