Contributions from the main river of the largest open sea delta in the Americas to the CO2 fluxes

Abstract

In this study, we sampled for the first time the main channel of the Parnaíba Delta, the largest open sea delta in the Americas, and two of its secondary channels, during the rainy season. Continuous measurements of pCO2, temperature, salinity, and wind velocity were taken, while subsurface water samples were collected to analyze for dissolved oxygen, pH, total alkalinity, dissolved organic carbon, and its isotopic, chlorophyll-a, and nutrients. The spatial variability of pCO2 along the different channels showed the existence of distinct drivers of CO2 dynamics in the area. The correlation of pCO2 in freshwater samples with dissolved oxygen and chlorophyll-a indicated the incidence of organic matter decomposition and primary production in the main channel and mangroves, while the highest salinity samples evidenced the control of carbonate equilibrium in the river mouth. Our data also indicated an important influence of the river discharge on the carbon dynamics of the estuary, with around 73% of the CO2 emissions in the estuary estimated to be from riverine CO2. The strong river effect in the estuary was also supported by the low salinities (0.04–26.37), pH (7.09 ± 0.36), and total alkalinity (328 ± 530.46 µmol kg−1 ), typical from fluvial waters. The estuary was supersaturated in CO2 and behaved as a strong source, with an average flux of 194.1 ± 135.1 mmol m−2 d −1.