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Abstract. Diatoms are key contributors to marine biogeochemical cycles, yet their distinctive reproductive strategies are rarely represented explicitly in ecosystem and biogeochemical models. In particular, the alternation between asexual size reduction and sexual size restitution introduces intrinsic size-structured dynamics that may influence plankton community structure and biodiversity. Here we incorporate a size-structured formulation of diatom reproductive dynamics into a coupled optical–biogeochemical plankton model to assess their impact on plankton biomass, diversity, and trophic interactions. The model represents size-dependent asexual reproduction as a shrinking flux across diatom size classes and sexual reproduction as a restitution process activated below a critical size threshold. Using long-term simulations and a Monte Carlo ensemble exploring uncertainty in reproductive parameters, we evaluate the sensitivity of plankton communities to variations in reproduction rates. Results show that asexual reproduction exerts a dominant control on diatom biomass and size structure, producing nonlinear, threshold-like responses capable of reorganizing phytoplankton functional composition and propagate upward through the food web. Sexual reproduction acts mainly as a secondary modulator, while trophic interactions buffer variability at higher trophic levels. These findings highlight reproductive dynamics as important regulators of plankton biodiversity and biogeochemical functioning.