DIEL CARBON DIOXIDE DYNAMICS IN A TROPICAL MANGROVE-DOMINATED DELTA

Estuarine CO₂ fluxes vary over diurnal and tidal cycles due to tidal pumping and ecosystem metabolism. Significant amounts of CO₂ in estuaries are primarily due to the freshwater input, the heterotrophic nature of the estuaries, and the additional lateral sources of carbon from coastal wetlands like mangroves. The carbon transfer from these wetlands to the estuaries can be a critical factor contributing to CO₂ variability in estuaries surrounded by such ecosystems. This study aimed to investigate the changes in pCO₂ over a tidal cycle in the Parnaíba River Delta. This delta is the largest in the Americas, located in a transitional climatic coast and mostly covered by mangrove forests. Two campaigns were conducted during the dry season. Two moored stations were established, one in an open bay (2019) and the other in a mangrove channel (2021). Continuous measurements were taken of pCO₂, temperature, salinity, and wind velocity during a 13-hour period. Additionally, subsurface water samples were collected every 2 hours for pH, total alkalinity, dissolved oxygen, and chlorophyll-a analysis at both stations. The station near the bay had lower salinity variability (35.5 – 36.6), compared to the one inside the mangrove channel (10.5 – 34.7). The pCO₂ varied between 423.7 to 766.9µatm inside the mangrove channel, and from 434.6 to 529µatm near the open bay, over the diel cycle. For the open bay station, pCO₂ was negatively correlated with salinity (R = –0.82, p<0.05) and varied with the tide and diel variability. In the mangrove channel, pCO₂ was higher only during low tide. Conversely, in the bay, pCO₂ is probably influenced by additional processes rather than tidal change. This study highlights the importance of considering tidal and diurnal variability when evaluating the CO₂ dynamics in estuaries, as the combination of diurnal and tidal processes may cause short-term fluctuations in pCO₂.