abstractThe Margalef´s mandala (1978) is a simplified bottom-up control model that explains how mixing and
nutrient concentration determine the composition of marine phytoplankton communities. Due to the
difficulties of measuring turbulence in the field, previous attempts to verify this model have applied
different proxies for nutrient supply, and very often used interchangeably the terms mixing and
stratification. Moreover, because the mandala was conceived before the discovery of smaller
phytoplankton groups (picoplankton <2 μm), it describes only the succession of vegetative phases of
microplankton. In order to test the applicability of the classical mandala to picoplankton groups, we
used a multidisciplinary approach including specifically designed field observations supported by
remote sensing, database analyses, and modeling and laboratory chemostat experiments.
Simultaneous estimates of nitrate diffusive fluxes, derived from microturbulence observations, and
picoplankton abundance collected in more than 200 stations, spanning widely different hydrographic
regimes, showed that the contribution of eukaryotes to picoautotrophic biomass increases with
nutrient supply, whereas that of picocyanobacteria shows the opposite trend. These findings were
supported by laboratory and modeling chemostat experiments that reproduced the competitive
dynamics between picoeukaryote sand picocyanobacteria as a function of changing nutrient supply.
Our results indicate that nutrient supply controls the distribution of picoplankton functional groups in
the ocean, further supporting the model proposed by Margalef.RADIALES (IEO
