The influence of phytoplankton size fractions on the carbon export ratio in the surface ocean

•We develop a metabolism-based mechanistic model of the carbon export ratio at the base of the mixed layer.•Our model delineates a lower bound on the carbon export ratio as a linear function of biomass ratios between phytoplankton size groups.•Our new approach incorporates phytoplankton size fractions in the assessment of ocean carbon export from satellites. The fraction of primary production exported out of the surface ocean, also known as the carbon export ratio, is believed to be a function of phytoplankton size fractions. However, this relationship is often elusive in observations. Here, we explore this relationship by developing a metabolism-based mechanistic model of the carbon export ratio at the base of the mixed layer (efml). Our efml model is a function of phytoplankton size fractions, temperature, and light and nutrient availability in the mixed layer. Our model delineates a lower bound on efml as a linear function of biomass ratios between phytoplankton size groups, supporting observational data from three cruises in the Southern Ocean. Finally, we develop a remotely-sensed estimate of efml incorporating satellite estimates of phytoplankton size fractions. Models like the one presented in this study will benefit from the improved characterization of plankton communities with the upcoming hyperspectral satellite imaging. With the projected shifts in plankton ecosystem structure associated with climate change, projections of air-sea carbon fluxes will require an improved representation of the impact of phytoplankton size fractions on the carbon export ratio. Plain language summary: Photosynthesis in the surface ocean converts CO2 into organic matter, part of which is transferred to depth. The fraction of organic matter transferred to depth is believed to be related to phytoplankton size fractions. For example, an ecosystem dominated by large phytoplankton cells is believed to be more efficient at exporting carbon. However, this relationship is poorly resolved. Here, we develop a model to investigate the influence of phytoplankton size fractions on the fraction of organic matter transferred to depth. Our model is a function of phytoplankton size fractions, temperature, nutrient, and surface irradiance. Our model supports observations of a functional relationship between fraction of organic matter transferred to depth and phytoplankton biomass size fractions. Our model can be used to improve estimates of organic matter export to depth in the world'