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

TL;DR

This study introduces a new framework to assess how rising temperatures and pollutants jointly affect aquatic organisms by combining temperature metrics with chemical exposure.

Contribution

The novel framework integrates degree-days and chemical activity to normalize toxicity assessments under varying temperatures.

Findings

Toxicity differences at higher temperatures were shown to result from faster physiological aging, not increased chemical potency.

Metabolic indicators revealed energy depletion caused by pollutants, independent of temperature.

D° normalization enabled accurate toxicity comparisons across thermal conditions.

Abstract

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.

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