Lagoons suffer from eutrophication and coastal acidification due to weak hydrological and tidal forcings. A wide range of tropical lagoons are accompanied by productive blue carbon ecosystems such as benthic macroalgae and seagrass. How the blue carbon ecosystems affect the carbon cycle in tropical lagoons remains unclear. Through 7 field surveys in Xiaohai Lagoon during 2023-2024, we aim to 1) evaluate the riverine dissolved inorganic carbon (DIC) fluxes exported to lagoon, 2) reveal the spatial-temporal distributions and controlling mechanisms of carbon dioxide partial pressure (pCO₂), air-water CO₂ exchange flux and carbonate system, and 3) assess the status of coastal acidification. The results show that the surrounding rivers annually exported 6.3×10⁵ mol DIC and 5.4×10⁵ mol total alkalinity (TA) to Xiaohai Lagoon. The water in river channels as a whole is a strong source of atmospheric CO₂, with an annual mean flux of 339±300 mmol m^-2 d^-1. Lagoon waters were a source of atmospheric CO₂ in spring (15±20 mmol m^-2 d^-1) and summer (10±19 mmol m^-2 d^-1), but became a sink in autumn (-15±11 mmol m^-2 d^-1) and winter (-28±29 mmol m^-2 d^-1) with an annual mean flux of -4±25 mmol m^-2 d^-1. Based on two end-member mixing model and the black and white bottle oxygen incubation experiments, we reveal that the gradually restored benthic macroalgae/seagrass in Xiaohai Lagoon could maintain photosynthesis much stronger than microbial respiration from summer to early winter, leading to higher pH relative to the conservative mixing lines. Therefore, coastal acidification was not a major environmental concern in Xiaohai Lagoon during most of the year. Our study suggests that the gradually recovering blue carbon ecosystems are hopeful to further promote the self-restoration of the aquatic ecosystems in tropical lagoons.

CO2 flux, coastal acidification, tropical lagoon, seasonal variation, blue carbon ecosystem