Diel variability of bulk optical properties associated with the growth and division of small phytoplankton in the North Pacific Subtropical Gyre

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 mu m driving the daily cycle of particle production and loss within the mixed layer. Flow cytometry data reveal the nitrogen-fixing cyanobacterium Crocosphaera (similar to 4 -7 mu m) to be an important driver of c(p) at the time of sampling, whereas Prochlorococcus dynamics (similar to 0.5 mu 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. (C)2020 Optical Society of America.