Seasonal and Yearly Controls of CO2 Fluxes in a Tropical Coastal Ocean

Abstract

Abstract Carbon dioxide (CO2) flux from Earth’s surface is a critical component of the global carbon budget, and the ocean surface is a significant CO2 source and sink. The tropical coast absorbs CO2 due to phytoplankton abundance and the all-year availability of photosynthetically active radiation. However, the role of the tropical coastal ocean in the global carbon budget is uncertain because of its underrepresentation in the literature. This study is the first to describe the variations of long-term CO2 flux in the tropical coast on monthly and annual scales using the eddy covariance method and remote sensing data. The 5-yr average of the CO2 flux is −0.089 ± 0.024 mmol m−2 day−1, which indicates that it is a moderate carbon sink. The results show that the CO2 flux varied seasonally: the fall transitional, southwest, spring transitional, and northeast monsoons partitioned the flux into three phases: increasing, stable, and decreasing. The rising and falling stages can be identified by the erratic behavior of the flux, whereas the stable phase’s fluxes were relatively constant. The environmental parameters that regulated CO2 flux were chlorophyll a, sea surface temperatures, wind, and atmospheric stability, which modulated the CO2 flux on the monthly time scale. Wavelet analysis corroborated the finding and revealed the role of photosynthetically active radiation (PAR) on CO2 flux through El Niño–Southern Oscillation. On the monthly time scale, sea surface temperature only slightly affected the fluxes, unlike chlorophyll a, but temperature’s control on the flux became more apparent on the yearly time scale. These findings help us to understand the monthly and yearly controls of CO2 flux and could contribute to developing models for predicting the flux on the tropical coast.