Seasonal and spatial variability of CO 2 emissions in a large tropical mangrove‐dominated delta

This study quantified the seasonal and spatial variability of partial pressure of CO 2 ( p CO 2 ) and water‐atmosphere CO 2 fluxes in the Parnaíba River Delta, the largest delta in the Americas. It is a pristine equatorial, mangrove‐dominated environment located in a transitional between humid and semi‐arid climates, with marked seasonality in rainfall and river discharge. Major channels and bays were sampled during dry and wet seasons, with continuous measurements of p CO 2 , temperature, salinity, and wind velocity. Subsurface water samples were collected in discrete stations for pH, total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved oxygen and chlorophyll a quantification. A significant positive correlation between carbonate system parameters with salinity was found in both periods, with salinity significantly higher in the dry season. Strong deviations of p CO 2 , TA, and DIC from two endmembers conservative mixing were found, particularly in mangrove‐dominated waters, due to organic matter degradation. The Delta showed high spatial variability of p CO 2 , with the highest values in mangrove‐dominated waters, moderate in the river‐dominated regions, and lowest in the high salinity areas, suggesting that p CO 2 variability is likely controlled by a combination of river‐ocean mixing and biological processes (respiration and photosynthesis). The Delta outgasses about 20 times less CO 2 in the dry season (9.06 ± 11.09 mmol m −2 .d −1 ) than in the rainy season (209.68 ± 250.87 mmol m −2 d −1 ). Our results indicate this large mangrove‐dominated tropical delta is an important source of CO 2 to the atmosphere, but a sharp decrease was observed during dry periods.