New Method for Quantification of Phenotypic Plasticity Reveals How Plasticity Changes Over Time in the Diatom <scp> <i>Thalassiosira weissflogii</i> </scp>

Quantifying phenotypic plasticity, the capacity of organisms to adjust phenotypes in response to environmental changes, is essential for understanding ecological and physiological resilience under climate stress. However, existing methods often lack flexibility and precision, especially under multi-dimensional environmental conditions. Here, we introduce a novel statistical approach, the Environmentally Standardized Plasticity Index (*ESPI), which integrates Hedges' <i>g</i> for effect size quantification and Euclidean distance for characterizing environmental variability. We validated this method using both simulated datasets and empirical data from the marine diatom <i>Thalassiosira weissflogii</i>, investigating five key phenotypic traits over 7 days under varying temperature, irradiance, and nutrient conditions. Our findings indicate distinct temporal patterns of plasticity: certain traits, such as photosynthetic efficiency (alpha) and saturation irradiance (<i>E</i> _k), demonstrated high initial plasticity followed by gradual acclimation, whereas others, like pigment composition, exhibited delayed phenotypic responses. This temporal dimension highlights the critical role of the growth phase in shaping plasticity responses. The proposed *ESPI method provides a robust, intuitive, and versatile framework for quantifying phenotypic plasticity, offering significant advances in predicting organismal adaptation to environmental change.