High-density sampling reveals biologically and tidally driven spatiotemporal variation in carbon dioxide fluxes in a tropical lagoon

Abstract

Lagoons are recognized as significant CO2 sources in the global carbon cycle. However, the lack of comprehensive measurements capturing simultaneous spatiotemporal variations in partial pressure of CO2 (pCO2) limits our understanding of mechanisms driving CO2 dynamics in lagoons. In this study, autonomous buoys were deployed at five stations across Chiku Lagoon (Taiwan) during the wet season, continuously measuring temperature, salinity, and pCO2 for over 24 h. Discrete water samples of total alkalinity and dissolved inorganic carbon were collected to establish a freshwater-to-seawater mixing model. Our analysis revealed that biological activity accounted for most of the pCO2 variability (59%), followed by mixing processes (36%) and the temperature variations effect (5%). These effects contributed to spatial heterogeneity, with the upper lagoon exhibiting stronger emissions (4.8 +/- 2.5 mmol m-2 h-1; mean +/- standard deviation) than the middle and lower lagoon (0.6 +/- 0.8 to 1.4 +/- 1.3 mmol m-2 h-1). Meanwhile, tidal influences on CO2 fluxes were evident, with emissions at low tide (1.4 +/- 0.5 mmol m-2 h-1) nearly double those at high tide (0.6 +/- 0.3 mmol m-2 h-1). On average, all stations acted as net sources of atmospheric CO2 over the sampling period (1.2 +/- 1.2 mmol m-2 h-1). A resampling sensitivity test of the high-resolution buoy data suggests a 3-h interval is optimal in biologically and tidally driven lagoons such as Chiku. These results provide a framework for understanding spatiotemporal CO2 dynamics and serve as a guide for future monitoring and carbon management strategies in coastal environments.