High-Density Sampling Reveals Seasonal Spatiotemporal Variations in Partial Pressure of Carbon Dioxide in a Tropical Lagoon

Lagoons are recognized as net sources of carbon dioxide (CO₂) to the atmosphere, with pronounced spatial and diurnal variability in partial pressure of CO₂ (pCO₂) and air–water CO₂ fluxes. Furthermore, these spatiotemporal variabilities are affected by seasonal weather changes associated with the terrestrial inputs from nearby human activities on land. Such dynamic pCO₂ variations rely on a high-density sampling strategy, with five to six lab-made CO₂ buoys deployed for over 24 hours across Chiku Lagoon, Tainan, Taiwan, measuring water temperature, salinity, and pCO₂ every minute. Four field campaigns were conducted during January 2022, April 2023, August 2020, and September 2021 to investigate the seasonal variability. This high-density sampling strategy has revealed pronounced pCO₂ changes among four campaigns, with the highest average pCO₂ value in August 2020 (1931±980 μatm) and the lowest average value in April 2023 (732±228 μatm). Across all sampling periods, the lagoon acted as a net source of atmospheric CO₂ (1.3±1.4 mmol m^–2 h^–1), with the strongest average emission in August 2020 (1.9±3.2 mmol m^–2 h^–1), which was twice higher than the average emission in April 2023 (0.9±1.2 mmol m^–2 h^–1). Through analyzing pCO₂ deviations from a two end-member mixing model, shifting between biological activity (photosynthesis and respiration) and tidal-induced mixing processes were revealed across seasons. In August 2020, biological activity was the dominant factor on pCO₂ changes, while the mixing effect and biological activity both controlled pCO₂ changes in January 2022 and April 2023. Additionally, Chiku Lagoon was found to act as a CO₂ source while functioning as a net autotrophic system in August 2020. These findings underscore the necessity of high-density sampling to resolve rapid and dynamic carbon cycling in tropical lagoons across diurnal, spatial, and seasonal scales, thereby providing a foundation for regional environmental management and offering strategies to assess the carbon footprint and enhance carbon neutrality in local industries.