The atmospheric concentration of the potent greenhouse gas methane has almost tripled since the industrial revolution. Approximately 50% of total global methane emissions can be attributed to aquatic ecosystems, which are increasingly altered by anthropogenic land use change. Intermittent streams are a common feature of coastal floodplains and have the potential to emit higher quantities of methane compared to perennial rivers, especially during stagnant water conditions, which can be amplified by land use. However, there have been no methane measurements in these systems.
We investigated the impact of land use (low disturbance forest, moderate disturbance cropping, and high disturbance perennial horticulture) on seasonal and diurnal methane concentrations, fluxes, and processes underlying methane production in nine subtropical intermittent streams in the coastal region of northern New South Wales. In dry periods soil- atmosphere methane fluxes were measured by chamber incubations on the dry stream bed and during wet periods when there was water flowing, we estimated water-atmosphere methane fluxes from gas concentrations in the stream water.
Initial methane data suggests that streams with high disturbance in their catchment have elevated methane emissions compared to streams with less disturbed catchments. During the summer wet period (November 2022), at day and at night, high disturbance streams had higher methane concentrations (6.06 ± 0.53 μmol L-1 (day), 7.61 ± 0.18 μmol L-1 (night)) than low disturbance streams (4.11 ± 0.39 μmol L-1 (day), 3.62 ± 0.23 μmol L-1 (night)), despite relatively similar dissolved nutrient concentrations. This might result from lower daytime water temperatures at shaded forest sites (20.2 – 23.6 °C) compared to agricultural sites without shading (30 – 31.7 °C) as well as higher light availability. This project will help to refine regional and global methane budgets of freshwater ecosystems and contribute to a better understanding of underlying processes of methane production.