Phytoplankton bottle samples collected in 1964 by R R. S. DISCOVERY Among the International Indian Ocean Expedition in the West Indian Ocean were studied, Samples were concentrated on milllipore filters. All species were identified as far as possible and counted. 237 species were identified from the 59 samples.

The samples were taken from 6 different water masses: Tropical surface water; the South Equatorial current; South Equatorial subsurface water; mixed water from the equatorial undercurrent; North Equatorial subsurface water and Arabian Sea water. Each has characteristic temperature/salinity/nutrient ranges.

The millipore filter samples provided quantitative phytoplankton data from which different population parameters were estimated. Phytoplankton abundance was found to be largely dependent on nutrient level. Regions of high abundance were associated with environmental instability in the upwelling region off the Arabian coast; in the shear zone region at 10⁰S and at the boundary of the equatorial undercurrent. The overall abundance was low. Species diversity estimates were made. Species diversity was found to increase with nutrient level, largely due to an increase in the species component of diversity; and decrease in regions of fluctuating conditions caused mainly by a decrease in the equitability, component. Phytohydrographic regions were delimited using an adaptation of numerical taxonomic methods to classify the phytoplankton samples into groups on a basis of their phytoplankton content. These groups were found to the related to the water masses and ocean currents. Floral elements of associated species with similar distributions were derived, also using numerical taxonomic techniques. Co-ordinating these results it was possible to divide the sampling area into phytohydrographic regions with characteristic species abundance, diversity and floral elements. These different regions tended to be bounded by areas of increased phytoplankton abundance and decreased species diversity due to environmental instability in the boundary zone.