The interplay of phytoplankton competition and adaptation affects how
phytoplankton, and ultimately marine ecosystems, respond to global
warming. However, current ecosystem models do not consider both
processes simultaneously. To study how the interplay of competition and
adaptation affects phytoplankton responses to global warming, we
developed an innovative ecosystem model for the Baltic Sea that
simulates competition between three functional phytoplankton groups and
allows for adaptation to changing temperatures. We found that
competition and adaptation influence each other, with the outcome
depending on environmental conditions. In a steady environment,
competition drives adaptation to individual niches to reduce competition
pressure. In a changing environment, adaptation enhances the competition
pressure by allowing inferior competitors to mitigate the dominance of
pre-adapted superior competitors. Our results demonstrate that by
neglecting adaptation, models can overestimate warming-related changes
in species dominance. Ecosystem models should include both competition
and adaptation to accurately simulate phytoplankton responses to global
warming.