Plankton blooms in spring are largely driven by temperature-induced increases in cell division, a new study reveals. Nearly all life on Earth depends on the byproducts of photosynthesis, about half of which is produced by marine phytoplankton; thus understanding the nature of phytoplankton blooms is important for making predictions about global biogeochemical cycling in response to climate change. Yet, despite decades of study, the key driver of phytoplankton abundance remains debated. Here, Kristen Hunter-Cevera and colleagues analyzed data collected by an in situ device that monitored the cell division rate of Synechococcus, a common type of plankton. The device continuously collected data off the coast of Cape Cod for nearly 13 years. The data reveal a direct correlation between temperature and cell division rates. For each degree (in Celsius) increase of the mean temperature in April, the spring bloom advances four to five days. The authors also calculated the loss rate of phytoplankton, finding that it closely follows the division rate. Thus, they suggest that biological factors, such viruses and predators, are still waiting to consume the superabundance of plankton. These findings are highlighted in a Perspective by Alexandra Worden and Susanne Wilken.