Fugacidade do CO2, no delta do rio Parnaiba, Brasil

Abstract

Estuaries are important sources of CO2 to the atmosphere, emitting about 0.1 Pg C yr−1. This CO2 supersaturation occurs mainly due to the intense heterotrophy of these environments and discharge of CO2 enriched fluvial waters. However, there is an intrinsic variability to the use of drainage basins and the diversity of estuarine ecosystems, but there is a lack of data for the global carbon balance related to tropical equatorial environments. The Parnaíba Delta is the largest open ocean delta in the Americas, with its watershed being fundamental in the socioeconomic development of the region, offering from tourist potential to the richness of its biodiversity. The area sampled in the present study comprised 4 of the 5 delta channels, as well as some secondary channels. Real time samples of pCO2 were collected during the rainy season (April / 2017 and March / 2018). Also, 21 discrete samples were taken for analysis of dissolved carbon, total alkalinity, isotopic composition, chlorophyll-a and nutrients, together with the measurement insitu of dissolved oxygen, pH, temperature and salinity. The spatial variability of pCO2 between the different channels and bays sampled shows that there were distinct controls on the delta carbon dynamics. In 2017, along the main river channel and in secondary ones, DOC values together with the significant and strong correlation of pCO2 with dissolved oxygen (r = −0.81) and chlorophyll-a (r = −0.80) indicated that the processes of organic matter decomposition and primary production were predominant. The low salinity (Min = 0.03 Max = 17.51) found along the main channel shows the influence of the fluvial waters in the estuary during the rainy season, and the establishment of the estuarine gradient advancing towards the ocean. In 2018, along the delta channels sampled and at the mouth of the estuary , higher salinity values were observed, and the control of the carbonate equilibrium and the dilution of the marine water were significant in the pCO2 behaviour. These processes resulted in different fluxes of CO2 along the Parnaíba river delta. The mean flux of CO2 was 179.13 ± 210.38 mmol.m−2 .day−1, a value above that estimated for tropical estuaries in the southern hemisphere.