Insights of greenhouse gases (CO2, CH4 and N2O) dynamics in sub-tropical estuaries from a coupled hydrodynamic-biogeochemical estuarine model

Coastal waters are typically productive aquatic ecosystems and play an important role in the global greenhouse gas (GHG) budget. However, the uncertainty in the estimation of GHG emission from estuaries remains large due to significant variability in GHG concentrations in time and space. This study aimed to provide a more accurate estimation of GHG emissions from sub-tropical estuaries by validating and analyzing results from a 3D hydrodynamic-biogeochemical model used to capture the temporal and spatial dynamics of the major GHG (CO₂ CH₄, and N₂O). The model was applied to the Brisbane, Maroochy, and Noosa Estuary in Queensland, Australia, representing systems under high, median, and low human impacts, and was validated with datasets from long-term monitoring stations and field campaigns along the freshwater-marine continuum. Distinct spatial heterogeneity of GHG distribution was found with the upstream acting as a hotspot for emission to the atmosphere, despite this area occupying a relatively small portion of the rivers. Seasonal variations of pCO₂ at the surface were driven mostly by the changes in water temperature and DIC concentrations, while strong diurnal variation was also found, driven by the changes related to tidal forcing. All GHG showed distinct signatures in the three rivers, related to trophic statues and hydrology. The model allowed us to approximate the fraction of incoming carbon and nitrogen that was lost to the atmosphere as GHG emissions, which is a step towards improving regional and national GHG budgets. A link of the biogeochemical model to a parameter optimization software PEST is being used to assist in uncertainty analysis from the model outputs.