Phytoplankton plays a massively important role in the oceanic carbon cycling and biogeochemistry.
Despite its far-reaching importance, regional cross-ecosystem comparisons remain incomplete because
the data sets are often scattered and fragmented. Here we compiled and harmonized decadal scale
phytoplankton monitoring data sets from seven geographic regions of the world ocean, covering ca 45
thousand quantitative samples from European, North- and South American coastal waters. Nonmetric
multidimensional scaling revealed clear regional clustering of sampling locations, both when using
compositional relatedness or phylogenetic turnover of communities. Compositional and phylogenetic
relatedness of phytoplankton communities had a strong correlation with salinity and temperature gradients
(R2 ¼ 0.6e0.8). The regional taxon richness (S) varied by almost an order of magnitude, and scaled
with the ecosystem size (A) according to a power law: S ¼ 62 A0.35. The compositional turnover of
species (beta-diversity) was also positively related to ecosystem size, but also to mean regional salinity.