Abstract: Environmental change is characterized by the simultaneous action of multiple drivers, yet predicting biological responses under high environmental complexity remains challenging. Most experiments examine only a few stressors, leaving uncertainty about how population performance scales as the number of drivers increases. Here, we experimentally tested how increasing environmental dimensionality affects population growth in the marine diatom Thalassiosira weissflogii. Populations were exposed to a combinatorial set of environmental conditions created by manipulating seven drivers, including warming, elevated CO₂, light intensity, nutrient limitation, and metal stress, resulting in 127 unique environmental conditions representing all possible combinations of one to seven drivers. These 127 unique environmental conditions were independently cultured in 96-well microplates within plant growth chambers, where temperature, light intensity, and nutrient conditions were rigorously controlled. Population growth declined systematically as driver number increased, independent of specific driver identity, while extinction risk rose sharply with environmental complexity. Across the full driver space, growth responses were best explained by the presence of a single dominant stressor rather than additive or multiplicative accumulation of effects. However, consistent deviations from dominant- driver predictions revealed that additional drivers further intensified physiological stress. Global change drivers such as temperature and CO₂ modulated these patterns, sometimes buffering and sometimes amplifying stress, resulting in context-dependent predictability. Together, these findings demonstrate that environmental dimensionality itself constrains population persistence, while dominant stressors set physiological limits, highlighting both the utility and the limits of scalable frameworks for predicting phytoplankton responses to complex environmental change.

Key words: environmental complexity, global change biology, growth, marine environmental change, phytoplankton