Sulfide dynamics in the water of the Bahía Blanca Estuary and its relationship with physical and chemical variables.

Estuaries are highly dynamic systems where intense biogeochemical processes regulate the cycling of nutrients and redox-sensitive elements. However, information on reduced sulfur species in oxygenated estuarine waters remains limited. This study provides the first annual assessment of sulfide and sulfate dynamics in the surface waters of the Bahía Blanca Estuary (Argentina), a eutrophic but well-oxygenated coastal system influenced by natural variability and strong anthropogenic pressure. Samples were collected monthly between August 2023 and July 2024 at five sites spanning the inner to the outer estuary. Total sulfides, sulfates, dissolved nutrients, chlorophyll a, phaeopigments, and physicochemical parameters were measured to evaluate seasonal patterns. Sulfide concentrations ranged from 0.25 to 1.46 μM, with maximum values during summer and minimum in winter. Positive relationships between sulfides and turbidity indicate the potential role of suspended particles as microenvironments that favor sulfur transformations. Sulfate concentrations exhibited a clear seasonal trend, increasing during warm months and decreasing in winter, consistent with temperature-dependent solubility patterns. High ammonium concentrations, particularly in autumn, reflect both external inputs and internal recycling within this eutrophic estuary. Overall, this research demonstrates that detectable sulfide levels can occur in oxygenated estuarine waters and highlights the influence of organic matter, particle dynamics, and microbial activity on sulfur cycling. These findings provide the first baseline for sulfur dynamics in the Bahía Blanca Estuary and underscore the need for continued monitoring in human-impacted coastal environments.