Seasonal and spatial variability of CO2 emissions in a large tropical mangrove‐dominated delta

Abstract

AbstractThis study quantified the seasonal and spatial variability of partial pressure of CO2 (pCO2) and water‐atmosphere CO2 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 pCO2, 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 pCO2, 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 pCO2, with the highest values in mangrove‐dominated waters, moderate in the river‐dominated regions, and lowest in the high salinity areas, suggesting that pCO2 variability is likely controlled by a combination of river‐ocean mixing and biological processes (respiration and photosynthesis). The Delta outgasses about 20 times less CO2 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 CO2 to the atmosphere, but a sharp decrease was observed during dry periods.