Carbon dioxide hydrodynamics along a wetland-lake-stream-waterfall continuum (Blue Mountains, Australia)

The small-scale spatial variability in dissolved carbon dioxide (CO₂) and water-air CO₂ flux dynamics were investigated within first-order catchments of the upper Blue Mountains Plateau (New South Wales, Australia). Water samples were collected at 81 locations during winter and summer over two consecutive years across seven aquatic ecosystem types: wetland, impoundment, lake, tributary stream, mainstem, escarpment complex, and urban-aquatic interface. Dissolved [CO₂] ranged from 15 to 880 μM (94 to 4760%Sat), and dissolved [O₂] from 0 to 350 μM (0 to 101%Sat). CO₂ supersaturation was typically highest in wetlands and vegetated impoundments of the upper plateau, and decreased downstream approaching atmospheric equilibrium at the escarpment waterfalls. Gas transfer velocities ranged from 0.18 m d^-1 in lentic waters to 292 m d^-1 at the bottom of waterfalls due to bubble-mediated transfer. The first- and second-order streams represented only 4.8% of the total open water area yet contributed to 61% of the total water-air CO₂ outgassing. The lake, escarpment and mainstem group systems had narrow diel and seasonal CO₂ concentration variability, while wetlands and vegetated impoundments had the widest ranges. Our high resolution spatio-temporal sampling was essential to identifying CO₂ outgassing hotspots in these geomorphically diverse catchments. Overall, >95% of excess dissolved CO₂ traversing the upper Blue Mountains Plateau was outgassed to the atmosphere.