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

Abstract

This study quantified the seasonal and spatial variability of partial pressure of CO2 (pCO2) and water-atmosphereCO 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 markedseasonality in rainfall and river discharge. Major channels and bays were sampled during dry and wet seasons,with continuous measurements of pCO 2 , temperature, salinity, and wind velocity. Subsurface water sampleswere collected in discrete stations for pH, total alkalinity (TA), dissolved inorganic carbon (DIC), dissolvedoxygen and chlorophyll a quantification. A significant positive correlation between carbonate systemparameters with salinity was found in both periods, with salinity significantly higher in the dry season. Strongdeviations of pCO 2 , TA, and DIC from two endmembers conservative mixing were found, particularly inmangrove-dominated waters, due to organic matter degradation. The Delta showed high spatial variabilityof pCO 2 , with the highest values in mangrove-dominated waters, moderate in the river-dominated regions,and lowest in the high salinity areas, suggesting that pCO 2 variability is likely controlled by a combinationof river-ocean mixing and biological processes (respiration and photosynthesis). The Delta outgasses about20 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 animportant source of CO 2 to the atmosphere, but a sharp decrease was observed during dry periods.