Food quality mediates the temperature–size rule in a marine primary consumer: evidence from trophic biomarkers

The temperature-size rule predicts that rising temperatures lead to smaller adult sizes with important consequences for fitness, but how this response interacts with temperature-driven changes in food quality remains unclear. Fatty acids are critical indicators of good food quality and hold the potential to unravel these interactions, while stable isotopes can reveal metabolic responses. In a fully crossed factorial design, the benthic copepod <i>Tachidius discipes</i> was grown at 15 and 24°C and fed with fatty acid content varying diatoms resulting from culturing <i>Nitzschia</i> sp. at these respective temperatures. Our results show temperature and food quality impacted synergistically on copepods: body and clutch size decreased by 10% and 40%, respectively, at 24°C, with good-quality food alleviating the body size effect and increasing clutch size at 15°C. Increased copepod Δ¹³C, reflecting increased metabolic demands, was found at elevated temperatures and when fed with poor-quality food. Multiple regression models highlighted the importance of specific ω-3 and ω-6 fatty acids for body and clutch size, supporting our conclusion that metabolic and food quality-mediated responses to temperature rise resulted in energetic imbalances that mediated the interaction between food quality and the temperature-size rule with negative consequences for reproductive output.