The Murray-Darling Basin, Australia's largest river system, encompasses a vast area across several states, supporting agriculture and human populations. However, its river flows are highly variable, experiencing periods of drought and intermittent flows that can prevent connectivity between freshwater and marine ecosystems. The 2022 floods in the River Murray, Australia were the most significant in the past four decades, resulting in a substantial freshwater plume that extended 70 kilometres into the Southern Ocean. Here, we used stable isotope analyses (δ13C, δ15N, δ34S) to examine the influence of these inflows on the structure and function of the marine food web. We hypothesised that the input of terrestrially-derived organic matter would alter the isotopic values of organisms within the river plume. Sampling of pelagic and benthic fish and invertebrates within and outside the plume area during periods of varying river discharge was conducted. Preliminary results indicated that invertebrates within the plume exhibited depleted δ34S values, suggesting a significant freshwater influence. Furthermore, some fish species within the plume showed depleted δ13C values, reflecting the incorporation of terrestrially-derived carbon. Some crab species within the plume exhibited an enrichment in δ15N, potentially indicating a shift in trophic position, possibly linked to scavenging on invasive freshwater carp that emigrated with flood waters. These findings suggest that River Murray inflows significantly alter the isotopic values of marine organisms, indicating shifts in carbon sources and trophic dynamics within the plume. This study provides valuable insights into the ecological impacts of large freshwater inflows on the structure and function of the marine ecosystem.