Planktonic microorganisms must adapt quickly to environmental fluctuations such as nutrient depletion or light changes, but it is difficult to follow how discrete plankton communities respond to such challenges in situ. Ottesen et al. followed one plankton population along the Californian coast for 2 days using a robotic sampler attached to a free-drifting float. Repeated sampling combined with whole-genome transcriptome analysis revealed synchronous, distinct gene expression changes, particularly for genes involved in growth and nutrient acquisition pathways. Photosynthesis genes showed a 24-hour expression cycle, whereas heterotrophic substrate metabolism genes exhibited more temporally variable fluctuations. Across species, the changes observed in metabolic genes were remarkably similar, probably owing to general changes in nutrient availability. The detection of such robust expression patterns indicates that there is a well-regulated, rapid response to environmental cues and possibly even interspecies coordination in plankton communities.