<![CDATA[Background: Freshwater salinization is an emerging global stressor that disrupts osmotic regulation and energy balance in aquatic invertebrates. Although acute and chronic toxicity are traditionally evaluated separately, their biological relationship remains insufficiently integrated. This study aimed to determine whether salinity exposure in a laboratory-maintained population of Daphnia sp. is expressed as an acute–chronic response continuum, with physiological trajectories emerging across survival and reproductive endpoints. Methodology: Acute toxicity was assessed through a 24-hour sodium chloride exposure (0.00–3.80 g/L) to determine the LC50, followed by a 10-day chronic experiment using fractions of the LC50 (0–20%). Survival probability and reproductive onset were analyzed using Kaplan–Meier estimation and Cox proportional hazards models, while offspring production was evaluated using non-parametric tests. Findings: The 24-hour LC50 was estimated at 1.87 g/L. Chronic exposure significantly altered survival dynamics (χ² = 11.7, p = 0.02) and reproductive onset (χ² = 10.3, p = 0.04). Reproductive success declined from 100% in controls to 20–40% in exposed groups, while total offspring production differed significantly among treatments according to the Kruskal–Wallis test (χ² = 9.74, p = 0.045) and the Jonckheere–Terpstra test indicated a significant decreasing trend across the salinity gradient (JT = 79.5, p = 0.0148). Sublethal reproductive impairment occurred at concentrations far below the acute lethal threshold. Contributions: Salinity stress in Daphnia sp. operates along a continuous and time-dependent physiological trajectory, where reproductive disruption precedes mortality. Integrating acute lethality with chronic survival and reproductive responses provides a more ecologically relevant framework for freshwater salinity risk assessment]]>
