Shaded regions indicate nighttime hours. a Abundances (blue) and cell volume (green) of Prochlorococcus following a Lagrangian water mass in the upper mixed layer. b Abundances of virus-like particles (red), T4-like (orange), and T7-like (purple) cyanophages. Shaded regions indicate the 95% confidence intervals of cyanophage abundance measurements. c Percent of virally infected Prochlorococcus by T4-like cyanomyoviruses (orange), T7-like cyanopodoviruses (purple), and total cyanophage (black) determined with the iPolony method. Shaded regions indicate bounds of infection assuming infection was entirely synchronous, and cells were either in the late stages of infection (lower bound) or in the early stages of infection (upper bound). Dashed lines indicate the limits of accurate detection for cyanophage infection. Prochlorococcus cell size, abundance and cyanophage infection all had statistically significant diel periodicity (RAIN rhythmicity test, p-values < 0.001), whereas VLP, T7-like, and T4-like cyanophage abundances were not periodic (RAIN rhythmicity test, p-value = 0.62, 0.42, 0.40 for VLP, T7-like, and T4-like cyanophage, respectively).