Macrofauna is one of the animals that can be associated with seagrass beds, it can be used as an indicator to determine the quality of the waters. The purpose of this study was to determine seagrass importance index and know macrofauna community structures (density, diversity, uniformity, and dominance). The research was conducted in January 2018. The method used is purposive sampling with two observation stations, where each station is divided into five sub-stations, each sub-station is divided into five plots measuring observation 0,5x0,5m2. It also carried out measurements of physical and chemical environmental parameters (temperature, pH, salinity, and substrate type).
The results of study showed 73 species of macrofauna. While seagrass species found in this study consisted of 5 species. Thalassia hemprinchii is seagrass species with the highest importance value index of 156.8. Density and frequency value of highest macrofauna attendance was dominated by genus Cetirhium. Diversity index value (H ') macrofauna ranged from 3.11 to 3.19, uniformity index value (E) is 0.78, while the index of dominance was 1. In general, the base substrate of sand and sandy gravel, ranging 0-0,7m depth, water temperature ranged from 31.5 to 32.3°C, water pH ranges from 7.7 to 8.2, and water salinity ranges from 24-25o/ oo. Based on the category level of environmental pollution, water in Lagoi has not been contaminated with a high macrofauna diversity, stable communities, and there is no dominant species of the community. In general, physical and chemical conditions of the water environment can support life macrofauna organisms.
