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)

Abstract

The increasing concentrations of methane (CH 4 ) 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 CH 4 concentrations, identified major sources and calculated the fluxes at the airwater interface in an eutrophic tropical embayment, Guanabara Bay, Rio de Janeiro, Brazil. The bay presented high spatial variability of CH 4 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 CH 4 emissions. In most cases, high CH 4 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, CH 4 concentrations were not significantly higher in bottom hypoxic waters than in surface waters, suggesting that CH 4 diffusion from these sediments was modest, due to the prevalence of sulphate reduction over methanogenesis. Our calculated annual air-sea fluxes (565-980 lmol m 22 d 21 ) are well above those of most estuaries worldwide, showing that urban pollution can be an important source of CH 4 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 CO 2 -equivalent, CH 4 emissions reduced the net CO 2 sink in Guanabara Bay by 16%.

Methane Dynamics of Aquaculture Shrimp Ponds in Two Subtropical Estuaries, Southeast China: Dissolved Concentration, Net Sediment Release, and Water Oxidation

“…A clear temporal pattern was observed for the dissolved CH 4 concentrations in shrimp pond water with a much higher concentration at the final stage (Figure ), which might be related to the decrease in water salinity over the study period. We observed a significant and negative relationship between CH 4 concentrations and water salinity in both estuaries (Figure a), which was consistent with the results found in other estuaries (Cotovicz et al, ; Dutta et al, ). The atmospheric pressure in our study sites also greatly varied over the shrimp growth period (Table S1; P. Yang et al, ), with a similar temporal pattern as the CH 4 concentrations in water (Figure ).…”

“…Preweighed 60‐mL serum glass bottles were completely filled with water samples from an air‐tight water sampler that limited gas exchange and prevented the formation of gas bubbles (Abril et al, ; Cotovicz et al, ). To inhibit microbial activity, the bottles were sealed and 0.2 mL of saturated HgCl 2 solution was injected into each water sample (Cotovicz et al, ; Dutta et al, ). All water samples were stored in an ice cooler, transported back to the laboratory, and analyzed within three days of collection.…”

“…A headspace equilibration technique was used to determine dissolved CH 4 concentration. Preweighed 60-mL serum glass bottles were completely filled with water samples from an air-tight water sampler that limited gas exchange and prevented the formation of gas bubbles (Abril et al, 2007;Cotovicz et al, 2016).…”

“…To inhibit microbial activity, the bottles were sealed and 0.2 mL of saturated HgCl 2 solution was injected into each water sample (Cotovicz et al, 2016;Dutta et al, 2015). All water samples were stored in an ice cooler, transported back to the laboratory, and analyzed within three days of collection.…”

Anthropogenic inputs from a coastal megacity are linked to greenhouse gas concentrations in the surrounding estuary

“…Overall, the higher concentrations of CH 4 in sewage delivery areas in addition to the correlation between CH 4 and enterococci concentrations indicate that anthropogenic additions resulted in a higher baseline of CH 4 concentration (and likely flux) than would otherwise exist in an undisturbed HRE. The two estuaries with considerably higher CH 4 surface concentrations, Guanabara Bay and Pearl River estuaries, also have a history of extremely high anthropogenic inputs (Chen et al a ; Cotovicz et al ). CH 4 surface concentrations were greater, on average, in tributaries than in other sites, corroborating high values reported in small rivers and streams (Butman and Raymond ; McGinnis et al ; Sawakuchi et al ), especially those impacted directly by wastewater or agricultural inputs (Rajkumar et al ; Garnier et al ; Borges et al ; Drake et al ).…”