Acute and long‐term warming effects on the functional response and reproductive efficiency of marine copepods

Abstract The impact of climate change stressors on marine copepods, key organisms at the base of ocean food webs, remains understudied. This study examined how warming affects their functional and numerical responses, critical life history traits linked to fitness. We conducted laboratory experiments on the calanoid species Paracartia grani , exposing it to a +6°C increase at two temporal scales, short term (4 d) and long term (21 generations; ca. 1 yr), and compared with the control at 19°C. We observed distinct patterns between warming treatments. Short‐term warming increased intake rates and satiation levels, causing a mismatch between the functional and the numerical responses. Additionally, egg size and phosphorus body content declined, collectively resulting in reduced reproductive efficiency at high food concentrations. Long‐term warming shifted satiation to higher food concentrations, possibly due to decreased foraging capacity (i.e., maximum clearance rate) in the smaller, warm‐reared copepods; in turn, maximum feeding rates indicated physiological compensation, and reproductive efficiency showed no consistent pattern. High food availability altered the copepod elemental content, which appeared to be linked to reproductive effort. Short‐term warming raised C : P and N : P ratios, while C : N ratios remained stable across all treatments. Our findings reveal how warming and resource availability interact to shape copepod performance and elemental fluxes, and highlight the importance of incorporating time‐scale dependent thermal effects into ecosystem models for better forecasting in a context of climate change.