Sawa Lake is located at the eastern edge of Iraq's southern desert. It is a habitat for a variety of phytoplankton communities with limited knowledge about their diversity patterns and organization in literature. Thus, seasonal samples were collected from three sites (S1: south of the lake, St 2: in the middle, and St 3: north of the lake) during a period extending from autumn 2020 to summer 2021. Samples were subjected to a comprehensive study using different diversity indices, such as the Shannon diversity index, Simpson index, evenness index, richness index, and Jaccard index, to examine the organization and distribution of phytoplankton communities. A dominance of Bacillariophyceae was detected in the lake under study, followed by Cyanophyceae, Chlorophyceae, and Pyrrophyceae. The Simpson index values ranged from 0.86 to 0.64 during summer. The evenness diversity index was 0.021 in autumn and 0.011 in winter. The Shannon index was 2.31 in summer and 1.55 in autumn. Based on the Jaccard index results, the greatest degree of similarity (66.66%) was recorded between sites 1 and 3, while the lowest  (58%) was between sites 1 and 2. The richness index varied from 1.594 - 0.987 in winter and autumn, respectively. A positive correlation between the Shannon Index and the Sampson Index (r = 0.82) was detected at a significance level of P≤ 0.01 based on statistical analysis. Result indices were correlated with phytoplankton density, and it was found that the richness index represents the diversity better than other indices.