Assessment of carbon fluxes to coastal area during persistent drought conditions

Abstract

This study assesses the seasonal and hydrological factors controlling carbon fluxes and quality of organic matter through the maximum turbidity zone (MTZ) of a tropical semiarid estuary under extreme drought, the Jaguaribe River estuary located in northeast Brazil. Dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations decreased in the Jaguaribe River estuary during the past ten years, reflecting drought intensification and river damming. DOC and POC behavior were strongly correlated to the freshwater residence time and freshwater fraction. Correlations with Chl-a indicated that the biological activity did not regulate DOC but did POC. Stable isotope ratios (δ13C and δ15N) of POC showed that it was predominantly terrestrial-derived, suggesting mangrove as an important POC source. Dissolved inorganic carbon concentrations reflected the seasonal variability of the semiarid climate, being higher in the dry season due to the negative water balance during this period. The Jaguaribe River estuary’s MTZ behaved as a retainer of POC (-1.5 ± 0.2 × 104 tons.year −1), DOC (-1.0 × 105 tons.year −1) and DIC (-4.8 ± 3.3 × 105 tons.year −1) during the dry season and as an exporter of DOC (+ 2 x 104 tons.year −1) and DIC (+1.4 × 105 tons.year −1) in the rainy season. However, the reduced freshwater supply to the estuary resulted in carbon concentrations 50% lower than those reported in 2004 and carbon fluxes smaller than expected for a tropical region. This study indicates that, under a scenario of climatic changes, the intensification of droughts in NE Brazil can further increase carbon retention in the estuary, reducing its exportation to the ocean.