Light-driven forcing of cellular growth processes in the upper mixed layer of the ocean fuels primary and secondary production at the base of marine food webs. Motivated by high-frequency measurements taken by Lagrangian sampling in the North Pacific Subtropical Gyre (NPSG), we developed a hierarchical set of multi-trophic community ecology models to investigate and understand daily ecological dynamics in the near-surface ocean including impacts of light-driven growth, viral infection, and grazing on Prochlorococcus, the main primary producer in this ecosystem. Model-data fits reveal that integrating top-down processes with light-driven growth can jointly explain diel changes in cyanobacteria, (including infected cells), virus, and grazer abundances. Moreover, direct comparisons of in silico dynamics with in situ observations can be used to partition the relative contributions of viral-induced lysis and grazing to Prochlorococcus mortality. Model-based inferences estimate virusinduced lysis and grazing to contribute < 5% and > 90%, respectively, to overall Prochlorococcus mortality – providing further support for recent observations that viral-induced lysis is relatively _unimportant in shaping Prochlorococcus mortality in surface waters of the NPSG. While our results suggest grazing and viral-lysis explain 95% of Prochlorococcus mortality, our model-data integration also finds that a modest, but nonetheless statistically significant component of estimated Prochlorococcus mortality (5% with 95% confidence intervals ranging from less than 1% to 27%) was not attributed to either viral lysis by T4-or T7-like cyanophages or grazing by heterotrophic nanoflagellates. Instead, model-data integration suggests the ecological relevance of other mortality mechanisms in structuring Prochlorococcus populations in the NPSG – likely by mixotrophic nanoflagellates, but potentially including grazing by other predators, lysis by other cyanophages, particle aggregation, and stress-induced loss mechanisms. Altogether, combining mechanistic multitrophic models with high-resolution measurements provides a route to quantify the balance of bottom-up and top-down forces in the NPSG while identifying the potential for non-canonical factors to shape the structure and function of marine microbial communities.