AbstractAs a fundamental component for marine ecosystem functioning, phytoplankton require an appraisal of the extent of their adaptive potential to interpret the effects of natural or human-induced changes in marine habitats. To this aim, the study of the present ecological characteristics of phytoplankton species represents the first effort to assess their adaptive potential in a wide spectrum of environmental variables. Using a set of nine time series, this study aimed at describing the ecological niche of 10 worldwide-distributed species and testing whether individual species occupy the same niche across different environments. The species showed wide variations in frequency, average abundance and seasonal pattern among sites, while their annual maxima occurred over ample ranges of physical–chemical variables, with marked differences from site to site depicting a considerably wide global niche for each species. Although in some of the species this result can be attributed to the existence of multiple, morphologically similar taxa hardly identifiable in light microscopy, other cases could be explained by intraspecific diversity and/or enhanced adaptation potential in phytoplankton populations. Although probably not applicable to species with more restricted distribution ranges, these results should be considered when projecting present species distributions into a future scenario of climate change.