Geoecological disturbance and recovery in gravel-bed rivers after high-severity bushfire

Bushfires, as landscape-level disturbances, strongly impact river health. They are crucial to many ecosystems in Australia, but the increasing frequency of megafires has potentially devastating consequences. The Black Summer bushfires of 2019-2020 severely impacted many rivers on the Australian east coast. This study assesses the geoecological health of four gravel-bed rivers three years post-fire and investigates possible applications of the intermediate disturbance hypothesis (IDH), which is frequently applied to disturbances in terrestrial ecosystems. Geomorphology, water quality, macroinvertebrates, and microbes were assessed in the field and the laboratory as indicators of ecosystem health. Black Summer burn severity and extent varied among four selected subcatchments of Dignam’s Creek on the south coast of NSW. When compared to burn severity, microbial activity and select water quality metrics followed negative quadratic curves in line with the IDH. Other metrics followed positive quadratic curves, were linear, or were non-correlated with burn severity. Geomorphology did not differ except in the most severely burned site, potentially indicating an abiotic threshold effect. Macroinvertebrate community health did not differ, suggesting recovery or insignificant impacts from the bushfire. Applying the IDH to microbial activity and water quality metrics may provide insight into river recovery from bushfires and other disturbances.