In contrast to most seagrass meadows where seawater carbonate chemistry generally shows strong diel variations with a higher pH during the daytime and a lower pH during nighttime due to the alternation in photosynthesis and respiration, the seagrass meadows of the inner lagoon on Dongsha Island had a unique diel pattern with an extremely high pH across a diel cycle. We suggest that this distinct diel pattern in pH was a result of a combination of total alkalinity (TA) production through the coupling of aerobic/anaerobic respiration and carbonate dissolution in the sediments and dissolved inorganic carbon consumption through the high productivity of seagrasses in overlying seawaters. The confinement of the semienclosed inner lagoon may hamper water exchange and seagrass detritus export to the adjacent open ocean, which may provide an ideal scenario for sedimentary TA production and accumulation, thereby forming a strong capacity for seagrass meadows to buffer ocean acidification.