A study was conducted on the spatial distribution of marine plankton off-coast of Sitra, the Kingdom of Bahrain, from August 2014 – July 2015. Specifically, this research aims to determine and identify the planktons; categorize the plankton as phytoplankton and zooplankton and classify taxonomically; calculate the population size of the identified plankton based on abundance, density, and biomass. Find out significant differences in the calculated population size based on the parameter used; determine dispersion index of the identified plankton concerning spatial distribution. And monitor surface water temperature and salinity, its effect on the spatial distribution, and population size of the identified plankton. The collection of water samples was done using the towed method. Results showed 20 species of phytoplankton and 14 species of zooplankton. Of the phytoplankton, 13 species belong to Phylum Chrysophyta or diatoms, and 7 species in Phylum Dinoflagellata or dinoflagellates. The 14 species of zooplankton belong to 8 phyla- Protozoa, Cnidaria, Platyhelminthes, Rotifer, Annelida, Mollusca, Arthropoda, and Chordata. The identified species of phytoplankton and zooplankton vary in abundance, density, and biomass. Fragillariopsis and Navicula species are the most abundant phytoplankton with higher density and biomass, while Ceratium furca and Ceratium fusus for the zooplankton. Statistical analysis using ANOVA revealed significant differences at the .05 level in abundance and density. However, an insignificant difference in biomass. Based on the dispersion index, the phytoplankton and zooplankton are vertically distributed, and aggregation is mostly on the upper column where sunlight is abundant. Surface water temperature and salinity significantly affect the spatial distribution and population size of the identified marine plankton.

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