Drought, megafires and flood - climate extreme impacts on catchment-scale river water quality on Australia's east coast.

Increased frequency and intensity of drought, wildfires and flooding due to climate change has major implications for river water quality, yet there are limited high-temporal resolution data capturing the combined transient impacts of these extreme events at large catchment scales. We present flow-stratified water quality data from a large coastal catchment (Macleay River, Australia) spanning severe drought and extensive fires followed by flooding. We examine concentrations (C), discharge (Q) and flux of suspended sediment, major ions, dissolved organic carbon (DOC) and key nutrients (NO and PO), with a focus on the critical first-flush period after the fires. Highly elevated suspended sediment (∼5500 mg L; >100x median pre-fire levels) during the initial post-fire period reflected enhanced erosion from fire-impacted, high-relief landscapes, with peak monthly suspended sediment loads of ∼1.1-3.7 t ha. The greatest sensitivity to erosion was during initial flow events following fire, highlighting the compounding effect of sequential extreme events on sediment transport. Maximum solute concentrations typically occurred during the first hydrograph peak post-fire with significantly (P = 0.01) elevated major ions following the order of K>Ca>SO>HCO≈Mg>Cl>Na, broadly reflecting the composition of ash materials. Distorted CQ relationships for major ions, DOC and nutrients indicated mobilisation behaviour and enhanced surface runoff during initial hydrograph peaks post-fire, with mean concentrations and CQ relationships progressively shifting to those approximating pre-fire within ∼3-12 months. Elevated DOC (∼7x; P = 0.01) displays distinct changes in fluorescence excitation-emission matrix spectral characteristics, attributable to both fire and drought. Both NON (160 μM) and PO (7.5 μM) were significantly elevated after the fires (∼15-22x; P = 0.01), with maximum monthly loads of 0.82 and 0.14 kg ha respectively. Fast biogeochemical cycling of dissolved inorganic nitrogen (DIN) species occurred during initial flow events following fire, with NHN initially dominant (>80% of DIN) and exceeding ecosystem guideline threshold values (>100 μM NHN), followed by rapid (∼1 week) nitrification. The extreme dynamism and transience of water quality parameters highlights the critical importance of high-frequency sampling to adequately capture the compound impacts drought, fires and floods on aquatic systems.