Methane emissions from Australian estuaries

Estuaries link terrestrial, riverine and marine environments, where organic matter is biogeochemically processed, exported and exchanged with adjacent coastal ecosystems, the continental shelf and open ocean. Estuarine methane emissions have mostly been measured in high latitude systems in the Northern Hemisphere, with a focus on river-dominated estuaries with substantial river discharge and estuaries located near large population centres. Australia has the 5^th largest coastline and over 1,000 estuaries that are characterised by seasonally distinct rainfall regimes, associated unique freshwater and nutrient loadings, high evapo-transpiration rates, and low population densities, in contrast to their northern counterparts. To better understand the role of climate, hydrogeomorphology and anthropogenic disturbance in estuarine methane emissions, this study conducted underway measurements of dissolved methane and air-sea fluxes along 38 estuaries on the Southeast (New South Wales), North (far north Queensland, Northern Territory), and Southwest (Western Australia) coast of Australia during the austral summer season. The strongest controlling factor for estuarine methane emissions was anthropogenic disturbance, with modified estuaries having higher emissions than unmodified estuaries. However, the effect of disturbance differed in different climate and geomorphic groups. The highest methane emissions by climate were in the modified Köppen climate group Csb (806 ± 1669 µmol m^-2 d^-1) and by geomorphic type were in the modified lagoons (521 ± 713 µmol m^-2 d^-1). This study improves our knowledge of methane fluxes and their drivers in Australian estuaries, which in turn, improves global estimates of estuarine methane emissions.