Chemotaxonomy-based mapping of phytoplankton communities in the subtropical Southwestern Atlantic Ocean, with emphasis on the marine cyanobacterium Trichodesmium

•Distribution of phytoplankton taxonomic groups was associated with the water masses.•Tropical Water dominated the surface water at the Southwestern Atlantic Ocean.•Nano and pico-phytoplankton groups dominated in spring and autumn under TW domain.•There was agreement between microscopy and CHEMTAX estimates for Trichodesmium.•Myxoxanthophyll was not an adequate marker for CHEMTAX Trichodesmium estimates.