Carbon Dynamics in the water-atmosphere interface of the Parnaíba Delta, Brazil

Abstract

Coastal tropical systems, such as estuarine deltas, are a key component in the local and global carbon cycle, acting as pathways and reactors for the carbon on their route to the coastal ocean, and a potential source of carbon dioxide (CO2) to the atmosphere. However, many of these environments are still unaccounted for in global estimates and budgets, especially the ones in tropical and equatorial areas. Therefore, this thesis aimed to understand the spatial and seasonal variability of the partial pressure of the CO2 (pCO2) in the estuarine waters and the air-water CO2 fluxes, as well as the controlling biogeochemical processes, in the large mangrove-dominated Parnaíba River Delta (PRD), the largest open sea delta in Americas. The PRD is a tropical and remote delta, with extensive mangrove forest, located in a transitional climatic coast, with marked seasonality. A variety of channels and bays were sampled in four oceanographic campaigns, two during the rainy season (2017 and 2018), and two during the dry season (2019, 2021). Continuous measurements of pCO2, water temperature and salinity, and wind speed were taken, while subsurface water samples were collected along different stations for the analysis of pH, total alkalinity, dissolved oxygen, particulate and dissolved carbon, nutrients, pigments, fatty acids profile and isotopic composition. In addition, sediments, plankton, and vegetation samples were taken to evaluate the origin of the organic matter in the delta. The results showed that the seasonal variability of the pCO2 was mainly related to the precipitation, as higher values occurred during the rainy season, while during the dry season, the intense seawater intrusion significantly reduced the pCO2. There was significant spatial variability in the delta, with higher values of pCO2 in mangrove-dominated waters, moderate in river-dominated ones, and lower in marine-dominated regions. These differences indicate that the variability of pCO2 is controlled by a combination of processes: the mixture between river and ocean water due to the dilution of estuarine waters by seawater and/or the removal of organic matter by flocculation processes, and biological processes, such as photosynthesis and respiration. The presence of saturated fatty acids and bacterial markers indicates the intense mineralization of organic material, largely derived from the extensive mangrove forests in the region, which sustain the high pCO2. The PRD was a source of CO2 to the atmosphere, with fluxes around 20 times higher during the rainy season (210 ± 251 mmol.m-2. d -1) when compared to the dry one (9 ± 11 mmol.m-2.d-1). The results indicate that large tropical river deltas surrounded by extensive mangroves are important sources of CO2 to the atmosphere, but the presence of bays in the study area facilitated marine intrusion, showing that this process is an important factor in diluting continental emissions. The seasonal variability of fluxes should also be considered when estimating the annual contribution of CO2 by these types of systems, especially in the context of global climate change.