This dataset includes phytoplankton and simplistic protozooplankton analysis on seawater samples from the Chagos Archipelago, tropical Indian Ocean, May 2018. Cells were identified using light microscopy at x400 magnification. Cell dimensions were measured from photographs and biovolume was calculated following Sun & Liu (2003). A full description, including a guide to the data files, is given in file ReadMe.docx. Acknowledgement: Water samples were collected by Guy Stevens and Niv Froman from The Manta Trust, and by Tom Letessier from the Zoological Society of London, with funding from the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science.