Intensive water management of dryland rivers drives fine-sediment accumulation, redox-sensitive internal loading, and flow-controlled oxygen dynamics

The Menindee Lakes, situated on the lower Darling–Baaka River in central Australia, form a major regulated water-storage complex that supplies water to major agricultural and urban areas. As a shallow dryland lake–river complex, the system typically experiences prolonged low-flow periods punctuated by short pulse floods. However, the construction of the Menindee Lakes Scheme in the 1960s transformed the system into an artificial, low-energy lotic storage and sediment trap, fundamentally altering benthic sediment fluxes, residence times, and redox dynamics. With unexplained repeated mass fish mortality events over the past decade, it is essential to understand the biogeochemical mechanisms driving changes in oxygen availability.Here, we combined seasonal sediment core incubations, stable isotope measurements, and field data-driven dissolved-oxygen modelling to identify and quantify the transformations and fate of nutrients and redox-active elements. Intact sediment cores were incubated in the field at eight sites spanning hydrologically distinct regions, capturing a gradient from fine, organic-rich sediments upstream to sandier sediments downstream. A two-step sequential oxic-to-anoxic incubation design, applied to the same cores, quantified fluxes of nutrients and redox metals, as well as nitrate isotope dynamics (δ¹⁵N–NO₃⁻, δ¹⁸O–NO₃⁻), resolving key redox-driven transformations.Nutrient fluxes exhibited strong spatial and seasonal contrasts that aligned with flow regulation and associated fine-sediment accumulation. Fine-grained, organic-rich sediments associated with Lake Wetherell and the upper weir pool showed substantially higher biogeochemical reactivity than sandier downstream sites. In summer, weir-pool sediment oxygen demand nearly doubled, and Lake Wetherell consistently emerged as a biogeochemical hotspot, with NH₄⁺ and PO₄³⁻ release rates more than twice those elsewhere and PO₄³⁻ release increasing >20-fold. Under anoxic conditions, δ¹⁵N–NO₃ followed Rayleigh-type enrichment consistent with denitrification. However, δ¹⁸O–NO₃ showed decoupling from expected fractionation, indicating alternate redox-sensitive nitrogen cycling pathways (likely DNRA) that can recycle and retain N.Anoxic fluxes of reduced nitrogen and redox-active species from the weir pool were stoichiometrically converted to sediment oxygen demand (SOD), upscaled to the weir-pool scale, and incorporated into a dissolved-oxygen box model to quantify sediment-mediated oxygen demand under no-flow conditions and the flow required for recovery following re-oxygenation. This demonstrated that during no-flow drought conditions, SOD can accumulate rapidly, while recovery following re-oxygenation is sensitive to both the magnitude and duration of managed flow releases. By integrating field, laboratory, and modelling approaches, we demonstrate how flow regulation and management-driven fine-sediment accumulation control redox-sensitive sediment biogeochemistry and amplify seasonal oxygen stress in regulated dryland rivers.