Marine ecosystems in Earth System Models (ESMs) are greatly simplified, potentially neglecting key interactions between dynamic food webs, biogeochemistry, and climate change. We have developed an expanded marine ecosystem model that includes eight phytoplankton groups and four size classes of zooplankton, and is implemented in the Marine Biogeochemistry Library (MARBL) code base (MARBL-8P4Z).  Incorporation of more specific plankton types and size classes enhances the capacity to mechanistically represent important ecosystem and biogeochemical processes. MARBL-8P4Z simulations reproduce observed, global-scale patterns in biomass and community composition for both the phytoplankton and zooplankton. Pico-phytoplankton, group-specific, carbon biomass shows similar spatial patterns and magnitude as an in situ global dataset. The model simulates reasonable patterns of community composition with pico-phytoplankton groups and microzooplankton dominating biomass in the oligotrophic, subtropical regions (>50%), and nano-phytoplankton, diatoms and larger zooplankton groups dominating at higher latitudes and within upwelling zones. Plankton group-specific differences in elemental stoichiometry and export efficiency strongly impacts the spatial patterns and magnitude of carbon export by the ocean's biological carbon pump.  Properly accounting for these processes is key for projecting the impacts of climate change on marine ecosystems, biogeochemistry and associated climate feedbacks.