Phytoplankton fix 50% of the planet’s atmospheric carbon and underpin the world’s aquatic food chains and fisheries. Global change is altering the environments that phytoplankton experience: in rivers and coastal environments, light availability is decreasing due to higher nutrient inputs; while the warming of lakes and open oceans generate stronger thermoclines, leading to nutrient-poor conditions in well-lit surface waters. Such changes to temperature, light and nutrient regimes impact phytoplankton population dynamics; but their synergistic effects remain unclear. We explore how these climatic stressors interactively alter the covariance between key demographic parameters that underpin the resilience and resistance of phytoplankton populations: their intrinsic rate of increase (r) and carrying capacity (K). Using a meta-analytic approach, we compiled >1000 estimates of r and K for >100 species of phytoplankton estimated under different temperature, light and nutrient regimes. We show that although r and K are typically assumed to trade-off (negatively covary); different environmental combinations yield r-K covariances that vary from strongly negative to strongly positive. These findings provide new insights into the fate of phytoplankton populations in response to global environmental change – with important implications for predicting future disruptions to aquatic primary productivity, as well as food-web and algal bloom dynamics.