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

Carbon dioxide flux from the 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 under-representation 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 five-year average of the CO2 flux is −0.089 ± 0.024 mmol m−2 d−1, which indicate 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, which were the increasing, stable, and decreasing phases. The rising and falling stages can be identified by the erratic behavior of the flux, while the stable phase’s fluxes were relatively constant. The environmental parameters that regulated CO2 flux were chlorophyll-α, sea surface temperatures, wind, and atmospheric stability, which modulated the CO2 flux on the monthly timescale. Wavelet analysis corroborated the finding and revealed the role of PAR on CO2 flux through the El-Niño-Southern Oscillation. On the monthly timescale, sea surface temperature only slightly affected the fluxes unlike chlorophyll-α, but temperature’s control on the flux became more apparent on the yearly timescale. These findings help understand the monthly and yearly controls of CO2 flux and could contribute to developing models in predicting the flux on the tropical coast.