VARIABILIDADE ESPACIAL E SAZONAL DAS CONCENTRAÇÕES E EMISSÃO DE METANO (CH4) EM AMBIENTES AQUÁTICOS NA AMAZÔNIA CENTRAL: IMPLICAÇÕES PARA O BALANÇO REGIONAL DO CARBONO

Abstract

Methane (CH4) is the second most significant atmospheric greenhouse gas, surpassed only by water vapor, and it is approximately 25 times more effective than carbon dioxide in trapping heat. Methane is produced naturally through the decomposition of organic matter in anoxic environments and anthropogenically through activities such as rice cultivation and livestock farming. Inland waters contribute significantly as a natural source of CH4 to the atmosphere. Flooded areas account for approximately 55% of natural methane emissions. Rivers and lakes within the vast Amazonian ecosystem also serve as sources of CH4, although the dynamics differ between these two environments. Measurements of CH4 emissions and fluxes in Amazonian settings, particularly those evaluating seasonality and spatial variability, remain limited, with most studies concentrated in temperate zones. Research addressing these variations is crucial for understanding CH4 dynamics in the Amazon. This study aimed to: (1) estimate concentrations and diffusive fluxes of CH4 in floodplain rivers and lakes across different hydrological periods, including severe drought and flooding events; and (2) relate variations in CH4 concentrations and fluxes to environmental variables. Data were gathered from samples collected between 2005 and 2011 in the Negro and Solimões rivers, as well as from two sampling events during different hydrological periods (flood and recession) in Janauacá Lake, a floodplain lake in the Amazon. In the lake, samples were collected in 2017 from 28 spatially distributed points. For the rivers, concentration analyses employed the headspace technique, while fluxes were estimated based on concentration data and other variables. In Janauacá Lake, flux measurements were conducted using floating static chambers, with emissions calculated through regression between concentration and incubation time; concentrations were determined using an adapted equilibrator connected to a gas chromatograph. The Negro and Solimões rivers consistently exhibited supersaturated levels of CH4 relative to the atmosphere, with the Negro River typically showing higher concentrations (ranging from 0.004 µM to 0.223 µM) compared to the Solimões (ranging from 0.005 µM to 0.037 µM). Diffusive fluxes were also greater for the Negro River (0.09 to 8.6 mmol m-2 d-1) than for the Solimões (0.04 to 1.07 mmol m-2 d-1). Additionally, the highest concentrations were recorded during extreme drought events in the Negro River (0.22 µM). Regression models indicated that water depth and temperature could explain 70% of the variation in dissolved CH4 concentrations in the Negro River and 58% in the Solimões, highlighting the potentially strong effects of interannual hydrological variation on CH4 concentrations and subsequent releases in these rivers. In Janauacá Lake, CH4 concentrations ranged from 0.01 to 5.62 µM during the flood period, while during the recession, concentrations varied between 0.01 and 6.68 µM. No significant correlation was found between the density of flooded vegetation near the sampling points and CH4 concentrations (r = -0.22). In contrast to concentrations, the diffusive flux was higher during the recession (average of 5.392 mmol m-2 d-1) than during the flood (average of 0.997 mmol m-2 d-1). The two different types of aquatic environments studied here served as sources of CH4 to the atmosphere during sampling, each exhibiting distinct dynamics.