When Heat Meets Pollutants: Integrating Degree-Days and Chemical Activity Concepts for the Assessment of Temperature-Driven Toxicity

Aquatic organisms face simultaneous stress from rising temperatures and chemical contaminants, yet ecotoxicological assays rarely account for temperature-driven physiological aging. We introduce a framework combining degree-days (D°) and chemical activity to disentangle physiological and chemical drivers of toxicity. This approach was tested using <i>Daphnia magna</i> exposed to polycyclic aromatic hydrocarbons (PAHs). Two exposure designs were compared: (1) fixed duration (72 h) at 20 and 25 °C, and (2) D°-standardized exposure corresponding to 60 D° (72 h at 20 °C; 58 h at 25 °C). Chemical activity, a thermodynamic measure of bioavailable dose, was used as the exposure metric, with the median lethal activity (La50) as the primary endpoint. To provide a more nuanced view of the metabolism, sublethal responses (δ¹³C, C/N ratio, protein content) were also evaluated. In fixed-time exposures, La50 values were lower at 25 °C, indicating greater apparent toxicity at elevated temperature. This difference disappeared under D° normalization, showing that increased mortality reflected faster physiological aging rather than altered PAH behavior. Metabolic indicators supported this interpretation, revealing PAH-driven, temperature-independent energy depletion. By aligning exposure with cumulative thermal experience and quantifying dose as chemical activity, this framework enables temperature-normalized toxicity assessment and supports climate-aware ecological risk evaluation for HOCs.