  Planktonic communities are extremely diverse and include a vast number of
rare species. The dynamics of these rare species is best described by
individual-based models. However, individual-based approaches to planktonic
diversity face substantial difficulties, due to the large number of individuals
required to make realistic predictions. In this paper, we study diversity of
planktonic communities by means of a spatial coalescence model, that
incorporates transport by oceanic currents. As a main advantage, our approach
requires simulating a number of individuals equal to the size of the sample one
is interested in, rather than the size of the entire community. By theoretical
analysis and simulations, we explore the conditions upon which our coalescence
model is equivalent to individual-based dynamics. As an application, we use our
model to predict the impact of chaotic advection by oceanic currents on
biodiversity. We conclude that the coalescent approach permits to simulate
marine microbial communities much more efficiently than with individual-based
models.
