Diel population dynamics and mortality of Prochlorococcus in the North Pacific Subtropical Gyre

Marine ecosystem models often consider temporal dynamics on the order of months to years, and spatial dynamics over regional and global scales as a means to understand the ecology, evolution, and biogeochemical impacts of marine life. Large-scale dynamics are themselves driven over diel scales as a result of light-driven forcing, feedback, and interactions. Motivated by high-frequency measurements taken by Lagrangian sampling in the North Pacific Subtropical Gyre, we develop a hierarchical set of multitrophic community ecology models to investigate and understand daily ecological dynamics in the near-surface ocean including impacts of light-driven growth, infection, grazing, and phytoplankton size structure. Using these models, we investigate the relative impacts of viral-induced and grazing mortality for Prochlorococcus; and more broadly compare in silico dynamics with in situ observations. Via model-data fitting, we show that a multi-trophic model with size structure can jointly explain diel changes in cyanobacterial abundances, cyanobacterial size structure, viral abundance, viral infection rates, and grazer abundances. In doing so, we find that a significant component (between 5% to 55%) of estimated Prochlorococcus mortality is not attributed to either viral lysis (by T4- or T7-like cyanophage) or grazing by heterotrophic nanoflagellates. Instead, model-data integration suggests a heightened ecological relevance of other mortality mechanisms -- including grazing by other predators, particle aggregation, and stress-induced loss mechanisms. Altogether, linking mechanistic multitrophic models with high-resolution measurements provides a route for understanding of diel origins of large-scale marine microbial community and ecosystem dynamics.