This research aimed to examine the correlation between sediment fractions, phytoplankton biodiversity, and populations of blood cockles (Anadara granosa) in marine ecosystems. The study was conducted during February-April 2023 by collecting samples at Gambus Laut Beach, Batubara Regency, Indonesia. Parameters measured included sediment fraction, sediment organic matter content, phytoplankton, blood cockle population, temperature, current speed, water brightness, pH, salinity, and dissolved oxygen. Two sediment fractions were found, namely the sandy mud and muddy sand, and dominated by the muddy sand substrate. The percentage of gravel fraction ranged from 5.2-18.8, sand fraction 36.8-55.7, and mud fraction 25.5-57.1%. The organic material content was 9.9% (station 1), 10.8% (station 2), and 9.5% (station 3). The result established that twelve species of phytoplankton from the classes of Bacillariophyceae (ten species), Cyanophyceae (one species), and Xanthophyceae (one species) were found. Species of Chaetoceros sp., Isthmia sp., Pseudo-nitzschia sp., Skeletonema sp., and Oscillatoria sp. were found at all stations. The highest abundance of phytoplankton was at station 3 (861.1 cells/L), and the lowest was at station 2 (722.2 cells/L). Water quality parameters were relatively normal and support life in local waters. The statistical analysis results showed no significant correlation between phytoplankton abundance and sediment organic matter content and sediment type (gravel, sand, and mud), where the R values were 0.059, 0.171, 0.191, and 0.326, respectively. There was also no significant correlation between the abundance of blood cockles and organic sediment fraction (gravel, sand, mud), where the R values were 0.272, 0.290, 0.257, and 0.580, respectively.