Phytoplankton diversity, whether defined on the basis of functional groups or on the basis of numbers
of individual species, is known to be heterogeneous throughout the global ocean. The factors regulating
this diversity are generally poorly understood, although access to limiting nutrients and light is known
to influence distributions for certain groups of phytoplankton. Here, we develop a simple box model
of biomasses and a limiting nutrient to describe the composition of phytoplankton communities in the
euphotic zone. In addition to analyzing the relative importance of nutrient availability in generating and
maintaining diversity, we apply the model to quantify the potential role of zooplankton grazing and
ocean transport for the coexistence of competing species and phytoplankton diversity. We analyze the
sensitivity of phytoplankton biomass distributions to different types of grazing functional responses and
show that preferential grazing on abundant species, for example as formulated by the Holling type III
grazing function, is a key factor for maintaining species’ coexistence. Mixing and large-scale advection
are shown to potentially have a significant impact on the distribution of phytoplankton species and, in
general, enhance phytoplankton diversity. Based on the model solutions, we argue that ocean transports
of phytoplankton cells can have a significant influence on species composition and that even transports at
horizontal spatial scales of up to at least 102 km and vertical scales of 102m can be expected to be important
for sustaining phytoplankton diversity. The model is applied in a multi-species simulation (n = 200)
and model simulations suggest that global patterns of phytoplankton biodiversity are determined by a
few dominating species within each group. This finding is shown to be in accordance with ecological
observations