Cross-platform observing systems are requisite to capturing the temporal and spatial dynamics of particles in the ocean. We present simultaneous observations of bulk optical properties, including the particulate beam attenuation ( c p ) and backscattering ( b bp ) coefficients, and particle size distributions collected in the North Pacific Subtropical Gyre. Clear and coherent diel cycles are observed in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in c p and b bp to daytime particle growth and division of cells, with particles < 7 µ m driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera ( ∼ 4 − 7 µ m ) to be an important driver of c p at the time of sampling, whereas Prochlorococcus dynamics ( ∼ 0.5 µ m ) were essential to reproducing temporal variability in b bp . This study is a step towards improved characterization of the particle size range represented by in situ bulk optical properties and a better understanding of the mechanisms that drive variability in particle production in the oligotrophic open ocean.