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One group of organisms that can be used as bioindicators of water quality is benthos, these organisms are spread over several habitats and also in coastal areas, one of which is in the seagrass ecosystem on Bira Island, located in the Kepulauan Seribu Jakarta. The aim of this study was to determine the dynamics of benthic population as an illustration of the environmental conditions on Bira Island for 6 months of observation. The method used in this study was descriptive with survey design. Data collecting by purposive sampling with the transect method at 3 observation stations. The data were analyzed descriptively through the calculation of abundance, diversity and dominance. The results of the study identified 23 species of benthos included in the 3 phylum with the largest to lowest composition represented by molluscs, arthropods and echinoderms phylum. The benthic population fluctuates and shows a pattern of decreasing trends every month during observation. The abundance of the highest species of each phylum is represented by Turicula javana (Mollusc), Portunus sp (Arthropods) and Holothuria atra (Echinoderms. The Species Abundance based on the observation station shows that the highest abundance is at station 1, followed by station 2 and the lowest is station 3. Benthos diversity at each station is in the moderate category, and there is no dominant species