Impact of the catastrophic flood event in May 2024 on continental water input and dissolved nutrient dynamics on the Southern Brazilian Continental Shelf

Climate change is expected to lead to more frequent and intense flooding. The present study investigated the impact of a catastrophic flood event that affected southern Brazil in April and May 2024. Dissolved inorganic nutrients were evaluated through this continental influx to the Southern Brazilian Continental Shelf (SBCS) via estuarine plumes and Submarine Groundwater Discharge (SGD), revealing new insights into the effects of the flood disaster. We applied 223 Ra as an SGD tracer, together with physicochemical parameters, to assess these fluxes. Three main continental sources contributed to the shelf system: the Río de la Plata plume (RdP), the Patos Lagoon estuary (PL), and SGD, each supplying distinct macronutrient concentrations. The SGD was estimated as 5.5–35.8 × 10 8 in 2023 and 2.8–18.2 × 10 9 m 3 day −1 in 2024. In addition, the associated SGD nutrient fluxes exhibited a slight rise post-flood, which reached 3.4–22.1 × 10 8 for Si, 1.5–9.7 × 10 7 for DIP, and 1.5–9.9 × 10 6 mol day −1 for NO₂ − + NO₃ − . During 2024, the nutrient distribution over the continental shelf exhibited elevated concentrations, particularly for NO₂ − + NO₃ − and silicate, compared with previous studies. These variables showed a strong negative correlation with salinity, indicating continental input, in contrast to the pattern observed in 2023. Furthermore, LP contributed to an increased turbidity plume associated with landslides in northern headwaters, which affects the input of silicate and NO₂ − + NO₃ − , and high 223 Ra activity associated with these sediments. The total water discharge of the RdP estuary exceeded the annual average by 21%, potentially providing nutrient-rich contributions to the shelf. Overall, the flood event enhanced SGD and estuarine fluxes, altering the coastal nutrient balance.