Phytoplankton species counts from Atlantic Ocean surface water samples collected during JR15001 (AMT25) in September-November 2015.

This dataset contains phytoplankton abundances, as cell numbers per taxa, collected in surface samples along a transect on the Atlantic Ocean, during the 25th Atlantic Meridional Transect cruise (AMT25/JR15001). The samples were collected between September and November 2015 at 67 CTD stations, at 2 m or 5 m depth along a transect of the Atlantic Ocean between 50° N and 50° S. For each sampling site, two samples of 200 mL were put in amber glass bottles; one fixed with neutral Lugol's iodine solution (2% final concentration) and the other with formaldehyde (2% final concentration). Observations were carried out with a Zeiss Axiovert 200 inverted microscope. Cells > 10 µm were counted in a 50 mL chamber, following the Utermöhl method (Utermöhl, 1958). Species from the following divisions/classes were identified and counted from Lugol's-fixed samples: Bacillariophyceae (Diatoms), Dinophyceae, separated into Autotrophic (ADinos) and Heterotrophic Dinoflagellates (HDinos), Prasinophyceae; furthermore, the genus Phaeocystis and filamentous cyanobacteria belonging to three genera were also quantified. For the formaldehyde bottle, only coccolithophore species > 10 µm were identified and enumerated (due to coccolith degradation in Lugol's-fixed samples).