Abundance of microbial phytoplankton through the water column during the AMT21 (D371) cruise in September-November 2011.

This dataset contains depth profiles of the mean abundance of cyanobacteria and small (&lt;20 um) phytoplankton reported in cells per millilitre. Samples were measured using flow cytometry during the Atlantic Meridional Transect (AMT) cruise AMT21 (D371). The samples were collected between September and November 2011 from 200 m to the surface, using a Seabird CTD system with a rosette of 24 Niskin bottles of 20 L volume. Samples were collected from the predawn and noon CTDs and were stored in a refrigerator and analysed within 3 hours of collection. In total 938 discrete samples were analysed and quantified using a Becton Dickinson FACSort flow cytometer which characterised and enumerated <em>Prochlorococcus</em> sp. and <em>Synechococcus</em> sp. (cyanobacteria) and pico- and eukaryotic phytoplankton, based on their light scattering and autofluorescence properties. Data were analysed onboard the ship during the cruise.