1. Multiple anthropogenic changes, such as climate change and chemical pollution, threaten the persistence of natural populations. Yet, their potential additive and interactive effects on organismal performance and fitness are poorly understood, thus limiting our ability to predict the effects of the global change. 2. We conducted a laboratory experiment to study the singular and combined effects of experimental heat waves and micropollutants (i.e. low-concentration toxicants; henceforth micropollutants [MPs]) on the freshwater snail, Lymnaea stagnalis. To comprehensively understand physiological and ecological consequences of stress, we studied a broad range of traits from respiration rate to feeding performance and growth. We also determined traits contributing to fitness and immune responses, as these are key traits in determining both organismal fitness and interspecific (e.g. host-parasite) interactions. We tested whether a constant exposure to MPs affects the ability of snails to tolerate heat waves (8days of 23.5 degrees C), and subsequently to recover from them, and whether the effects are immediate or delayed. 3. We found strong immediate additive effects of both stressors on reproduction, while they synergistically increased respiration and antagonistically decreased food consumption. Moreover, these effects were transient. Although the heat wave increased metabolic rates, individuals did not increase their resource uptake. This caused an apparent imbalance in resource levelsa probable cause for the observed trade-off between immune function and reproductive traits (i.e. phenoloxidase-like activity decreased, while reproductive output increased). In addition, exposure to MPs led to a temporarily reduced reproductive output. 4. Our results indicate that even short-term heat waves and low concentrations of chemical pollution can have large, mainly additive impacts on organismal fitness (e.g. altering susceptibility to infections and reproductive output). This suggests that long-term effects of existing stressors and heat waves need to be considered when assessing the resilience of natural populations.