Identifying key indicator species, their life cycle dynamics and the multiple driving forces they are affected by is an important step in ecosystem-based management. Similarly important is understanding how environmental changes and trophic interactions shape future trajectories of key species with potential implications for ecosystem state and service provision. We here present a statistical modeling framework to assess and quantify cumulative effects on the long-term dynamics of the copepod Pseudocalanus acuspes, a key species in the Baltic Sea. Our model integrates linear and non-linear responses to changes in life stage density, climate and predation pressure as well as stochastic processes. We use the integrated life cycle model to simulate copepod dynamics under a combination of stressor scenarios and to identify conditions under which population responses are potentially mitigated or magnified. Our novel modeling approach reliably captures the historical P. acuspes population dynamics and allows us to identify females in spring and younger copepodites in summer as stages most sensitive to direct and indirect effects of the main environmental stressors, salinity and temperature. Our model simulations furthermore demonstrate that population responses to stressors are dampened through density effects. Multiple stressor interactions were mostly additive except when acting on the same life stage. Here, negative synergistic and positive dampening effects lead to a lower total population size than expected under additive interactions. As a consequence, we found that a favorable increase of oxygen and phosphate conditions together with a reduction in predation pressure by 50% each could counteract the negative effect of a 25% decrease in salinity by only 6%. Ultimately, our simulations suggest that P. acuspes will most certainly decline under a potential freshening of the Baltic Sea and increasing temperatures, which is conditional on the extent of the assumed climate change. Also the planned nutrient reduction strategy and fishery management plan will not necessarily benefit the temporal development of P. acuspes. Moving forward, there is a growing opportunity for using population modeling in cumulative effects assessments. Our modeling framework can help here as simple tool for species with a discrete life cycle to explore stressor interactions and the safe operating space under future climate change.