Ocean Acidification

Seaweed farming contributes substantial amounts of organic carbon to the ocean, part of which can be locked for a long term in the ocean and perform the function of ocean carbon sequestration, and the other part can be converted into inorganic carbon through microbial mineralization and aerobic respiration, affecting the pCO₂, pH_T and dissolved oxygen of seawater. It is generally believed that seaweed farming will cause the seawater to become a sink of CO₂ due to carbon fixation by macroalgal photosynthesis. However, little attention has been paid to the fact that seaweed farming environment may sometimes become a source rather than a sink of CO₂. Here, through in-situ mesocosm cultivation experiments and eight field investigations covering different kelp growth stages in an intensive farming area in China, we found that compared with the surrounding seawater without kelps, the seawater at the fast-growth stage of kelp was a sink of CO₂ (pCO₂ decreased by 17−73 μatm), but became a source of CO₂ at the aging stage of kelp (pCO₂ increased by 20−37 μatm). Concurrently, seawater pH_T experienced a transition from increase (by 0.02−0.08) to decline (by 0.03−0.04). In-situ mesocosm cultivation experiments showed that the positive environmental effects (i.e., pCO₂ decrease and pH_T increase) induced by kelps at the early growth stage could be offset within only 3 days at the late-growth and aging stages. The release of dissolved organic carbon by kelps at the late growth stage increased significantly, supporting the enhancement in microbial abundance and respiration, which was manifested by the remarkable decrease in seawater dissolved oxygen, ultimately leading to CO₂ release exceeding photosynthetic CO₂ absorption. This study suggests that mature farmed kelps should be harvested in time to best utilize their carbon sink function and environmental benefits, which has guiding significance for the rational management of seaweed farming.

Xiong T., Li H., Hu Y., Zhai W.-d., Zhang Z., Liu Y., Zhang J., Lu L., Chang L., Xue L., Wei Q., Jiao N. & Zhang Y., 2024. Seaweed farming environments do not always function as CO2 sink under synergistic influence of macroalgae and microorganisms. Agriculture, Ecosystems & Environment 361: 108824. doi: 10.1016/j.agee.2023.108824. Article.