Spatio-temporal variability of methane (CH ) concentrations and diffusive fluxes from a tropical coastal embayment surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil)

Abstract

The increasing concentrations of methane (CH4) in the atmosphere stress the importance of monitoringand quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014,we measured the spatial CH4concentrations, identified major sources and calculated the fluxes at the air-water interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presentedhigh spatial variability of CH4concentrations, without a significant trend with salinity, but observed theinfluence of the urban areas at its watershed. Although the more polluted sector of the bay accounts forabout 10% of the sampled surface area, it contributed to one half of the bay’s total CH4emissions. In mostcases, high CH4concentrations seemed be sustained by allochtonous sources such as the sewage network andpolluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at theinner and centre of the Bay, CH4concentrations were not significantly higher in bottom hypoxic watersthan in surface waters, suggesting that CH4diffusion from these sediments was modest, due to the preva-lence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565–980 lmol m22d21) are well above those of most estuaries worldwide, showing that urban pollution can be an importantsource of CH4to the coastal waters and even more significant than the presence of organic-rich environ-ments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO2-equiva-lent, CH4emissions reduced the net CO2sink in Guanabara Bay by 16%.