Spatio-temporal variability of methane (CH4) concentrations and diffusive fluxes from a tropical coastal embayment surrounded by a large urban area (Guanabara Bay, Rio de Janeiro, Brazil)

The increasing concentrations of methane (CH₄) in the atmosphere stress the importance of monitoring and quantifying the fluxes from coastal environments. In nine sampling campaigns between 2013 and 2014, we measured the spatial CH₄ concentrations, 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 presented high spatial variability of CH₄ concentrations, without a significant trend with salinity, but observed the influence of the urban areas at its watershed. Although the more polluted sector of the bay accounts for about 10% of the sampled surface area, it contributed to one half of the bay’s total CH₄ emissions. In most cases, high CH₄ concentrations seemed be sustained by allochtonous sources such as the sewage network and polluted rivers, especially under high accumulated precipitation conditions. In the most stratified area, at the inner and centre of the Bay, CH₄ concentrations were not significantly higher in bottom hypoxic waters than in surface waters, suggesting that CH₄ diffusion from these sediments was modest, due to the prevalence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565–980 μmol m−2 d−1) are well above those of most estuaries worldwide, showing that urban pollution can be an important source of CH₄ to the coastal waters and even more significant than the presence of organic-rich environments, like salt marshes and mangroves. Comparing the greenhouse gas emissions in terms of CO₂-equivalent, CH₄ emissions reduced the net CO₂ sink in Guanabara Bay by 16%.