Seasonal exports and drivers of dissolved inorganic and organic carbon, carbon dioxide, methane and δ13C signatures in a subtropical river network.

Riverine systems act as important aquatic conduits for carbon transportation between atmospheric, terrestrial and oceanic pools, yet the magnitude of these exports remain poorly constrained. Interconnected creek and river sites (n=28) were sampled on a quarterly basis in three subcatchments of the subtropical Richmond River Catchment (Australia) to investigate spatial and temporal dynamics of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC), carbon dioxide (CO2), methane (CH4), and carbon stable isotope ratios (δ13C). The study site is an area of high interest due to potential unconventional gas (coal seam gas or coal bed methane) development. DIC exports were driven by groundwater discharge with a small contribution by in situ DOC remineralization. The DIC exports showed seasonal differences ranging from 0.10 to 0.27mmolm-2catchmentd-1 (annual average 0.17mmolm-2catchmentd-1) and peaked during winter when surface water discharge was highest. DOC exports (sourced from terrestrial organic matter) had an annual average 0.07mmolm-2catchmentd-1 and were 1 to 2 orders of magnitude higher during winter compared to spring and summer. CO2 evasion rates (annual average of 347mmolm-2water aread-1) were ~2.5 fold higher during winter compared to spring. Methane was always supersaturated (0.19 to 62.13μM), resulting from groundwater discharge and stream-bed methanogenesis. Methane evasion was highly variable across the seasons with an annual average of 3.05mmolm-2water aread-1. During drier conditions, stable isotopes implied enhanced CH4 oxidation. Overall, carbon losses from the catchment were dominated by CO2 evasion (60%) followed by DIC exports (30%), DOC exports (9%) and CH4 evasion (<1%). Our results demonstrated broad catchment scale spatial and temporal variability in carbon dynamics, and that groundwater discharge and rain events controlled carbon exports.