Temperature is a fundamental driver of biological dynamics, but we do not know how it shapes the physiology and ecology of any community across global temperature gradients. Here I examine the influence of temperature on phytoplankton, which are extremely sensitive to changes in environmental conditions and play a critical role in global food webs and biogeochemical cycles. I address how global variation in temperature regimes has shaped distributions of phytoplankton temperature traits, identifying patterns of adaptation as well as differences in how major functional groups respond to environmental temperature gradients. I also show that due to the asymmetric cost of exceeding the optimal temperature and the traits of tropical species, ocean warming this century may drive a reduction in the diversity of tropical phytoplankton communities in the absence of evolutionary adaptation. Tropical phytoplankton species may persist, however, by poleward migration, bringing them into competition with temperate species. Our study of the temperature traits of an invasive cyanobacterium supports the idea that rising temperatures will increase the probability of invasion by tropical and subtropical species into temperate environments. Predicting these invasions, however, is a challenge that requires us to model the phytoplankton community dynamics in complex natural environments. This will require a mechanistic understanding of how temperature interacts with important resources such as nutrients and light to influence growth. To address this, I have developed and tested a model describing how temperature and nutrients interact to affect growth rates. Our experimental tests confirm a novel prediction: that optimum temperature for growth is a saturating function of nutrient concentration. Together, this work forms a foundation from which we can build predictive models of how environmental warming will affect population and community dynamics across broad spatial scales.
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