‘Hot spot’ v ‘hot moment’ regulation of catchment-scale aquatic greenhouse gas (CO2, CH4, N2O) emissions

Abstract:

The discrete reactive zones and events that regulate carbon and nitrogen fluxes from land to sea also likely regulate aquatic carbon (CO₂, CH₄) and nitrogen (N₂O) greenhouse gas emissions. But while increasing attention is paid to small inland waterways as greenhouse gas emission ‘hot spots’, quantifying emissions due to stochastic ‘hot moments’ remains challenging. Here we aimed to redress this gap by combining empirical dissolved N₂O, CO₂, and CH₄ measurements, spatial mapping, and hydrodynamic modelling to assess total emissions from across a lowland aquatic landscape. Sites included farm drains, ponds, creeks, rivers, and the receiving coastal waters across the Peel-Harvey catchment in Western Australia. The estuary accounted for 87% of the catchment’s aquatic surface area, but only 30% of emissions. In contrast, small farm drains (<1% of the aquatic surface area) produced 20% of emissions. However, a winter storm, when high winds (~100 km h^-1) increased estuary greenhouse gas emissions 40x, inverted this balance. Increased emissions were due to physical factors: wind-generated turbulence increased gas transfer velocities and enhanced porewater flushing (CH₄ concentrations increased from 2 μM to 30 μM during the highest wind period). This suggests a split between production-driven hot spots and emission-driven hot moments. Greenhouse gas emissions from aquatic networks will thus depend on both drainage system management and future storm frequency and intensity.