The response and recovery of the dissolved organic carbon cycle in ephemeral streams to large flood events

The supply, transport, and cycling of dissolved organic carbon (DOC) within river systems is a critical component of the carbon cycle, determines freshwater ecosystem primary productivity, and is a key driver of water quality through associated redox transformations. Despite this importance, there is very little understanding of how both the concentration and quality of DOC are influenced by natural variations in hydrology, particularly extreme events such as floods, and within different climatic zones. In this study, we examined the evolution of DOC within an ephemeral catchment in semi-arid Australia during a 1:20 year flood event. Total DOC concentrations increased ~3 fold compared to pre-flood concentrations, with peak concentrations occurring in the rising limb of the hydrograph, similar to previous studies of event based DOC. Although we found the DOC to be dominated by Humic substances, the behaviour of the Humic fraction more closely followed the flood hydrograph, suggesting there is a contrast between the large pool of DOC that can be released quickly at the beginning of an event, and DOC which is transported and mixed within the actual flood wave. We also analysed the fluorescence characteristics of the dissolved organic matter, which provides insights into the ability of micro-organisms to process DOC within floods, and also how these systems respond during flow recession. In terms of the total DOC budget, this one event accounts for ~ 6 times the mean annual DOC export of this catchment, within 2% of the time. This work extends our knowledge of riverine DOC fluxes to ephemeral environments, and highlights the potential importance of extreme events to ecosystem carbon processing.