  Seed formation is part of the reproductive cycle, leading to the accumulation
of resistance stages that can withstand harsh environmental conditions for long
periods of time. At the community level, multiple species with such
long-lasting life stages can be more likely to coexist. While the implications
of this process for biodiversity have been studied in terrestrial plants, seed
banks are usually neglected in phytoplankton multispecies dynamic models, in
spite of widespread empirical evidence for such seed (or rather cyst) banks. In
this study, we build a metacommunity model of interacting phytoplankton species
with a cyst bank. The model is parameterized with empirically-driven growth
rate functions and field-based interaction estimates, which include both
facilitative and competitive interactions. Exchanges between compartments
(coastal pelagic cells, coastal cysts on the seabed, and open ocean pelagic
cells) are controlled by hydrodynamical parameters to which the sensitivity of
the model is assessed. We consider two models, i.e., with and without a
saturating effect of the interactions on the growth rates. Our results are
consistent between models, and show that a cyst bank allows to maintain all
species in the community over 30 years. Indeed, a fraction of the species are
vulnerable to extinction at specific times within the year, but this process is
buffered by their survival in the coastal cyst bank. We thus highlight the
potential role of the cyst bank in the recurrent re-invasion of the coastal
community, and of coastal environments in re-seeding oceanic regions. Moreover,
the cyst bank enables populations to tolerate stronger interactions within the
community as well as more severe changes in the environment, such as those
predicted within a climate change context. Our study therefore shows how a cyst
stage may help phytoplanktonic diversity maintenance.
