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 Daphnia magna 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.