Fire and flood: Impacts along the Australian river-to-ocean aquatic continuum

Dylan K. Laicher-Edwards

doi:10.25918/thesis.448

In 2019, Australia experienced its hottest year on record, exacerbating the worst bushfire season in colonised history. Intense rainfall eased conditions, leading to significant erosion and runoff of fire-related materials into critical coastal aquatic systems, particularly in New South Wales (NSW). Existing research has dominantly explored the impacts of wildfire to inland freshwater environments, the effects along the river-to-ocean continuum remain largely unexplored, despite their importance in a future with increased extreme precipitation events. This thesis sought to investigate fire-flood impacts along this continuum, examining nutrient, metal, and carbon dynamics in various coastal aquatic systems in NSW. The study examined dissolved organic carbon (DOC) and dissolved black carbon (DBC) concentrations in six Intermittently Closed and Open Lake or Lagoons (ICOLLs), which due to their hydrology and extended periods without flushing, were expected to show a more distinct fire-signal compared to macrotidal estuary types. Surface and shallow groundwater samples were analysed for DOC and DBC by solid phase extraction and BPCA analysis, revealing that smaller ICOLLs exhibited Higher DBC concentrations in surface waters, with shallow groundwater potentially acting as a significant DBC source over time. The study additionally explored delayed fire impacts to 13 fire-affected estuaries and results indicated a significant increase in total dissolved phosphorous in all estuaries, as well as particular metals, such as zinc and arsenic, occurring above guideline values several months following the fires and floods. These findings underscored the need for comprehensive pre-, during-, and post-fire data to assess impacts accurately. Further, the study assessed the potential of bivalves as proxies for tracking trace metal trends in fire-impacted systems. LA-ICP-MS analysis of Hyridella drapeta shells from a fire-impacted coastal river system revealed significantly elevated manganese levels during the fire-flood period and results suggested manganese to be a potentially strong indicator of post-fire and flood conditions in upstream coastal river systems. Overall, the research demonstrated in this thesis suggests that high-severity fires can have prolonged impacts on smaller, less frequently flushed ICOLLs compared to river-dominated systems. It also highlights the importance of understanding the unique dynamics of each estuary and catchment to accurately predict and mitigate post-fire impacts along the river-to-ocean continuum.

Fire and flood: Impacts along the Australian river-to-ocean aquatic continuum

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Fire and flood: Impacts along the Australian river-to-ocean aquatic continuum

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