Abstract: The epilithic algal matrix (EAM) is widely distributed in coral reef ecosystems and plays a crucial role in key processes such as primary production, nutrient recycling, sediment deposition, and coral reef phase shift. In order to investigate the morphological characteristics and distribution patterns of EAM, a survey was conducted in the typical fringing reef of Sanya in January, April, July, and October 2022. The survey consisted of three sections (1m, 3m, 6m). The substrate types and their coverage were recorded by video, and three substrate types (branched, massive, planiform) with epiphytic algae were sampled by scuba diving. The results showed significant spatial differences but no seasonal differences in the coverage of EAM in the study area. The coverage rate was higher at depths of 1 and 6 meters, but lower at a depth of 3 meters, with averages of 66.96% and 16.55%, respectively. The coverage of EAM showed a negative correlation with the live coral coverage rate. The EAM is mainly distributed on hard substrate surfaces with high porosity and has the highest coverage degree.98 % of the dead planiform coral reefs appeared. However, the EAM on the dead massive coral reefs exhibited the highest algal height (11.16 ± 0.68mm), biomass (118.51 ± 33.64g∙m^-2), and organic matter content (102.49 ± 32.94g C∙m^-2). This could be attributed to the relatively high surface porosity of the dead massive coral reefs. The morphological characteristics and biomass of EAM vary with depth and season. The highest value is observed at a depth of 3 meters, while the lowest value is found at a depth of 1 meter. In terms of time, the height, density, and biomass of EAM were highest during the summer and lowest during the winter. The results here show that the benthic community in the studying area of Luhuitou, Sanya consists mainly of long sediment-laden algal turfs (LSATs) with high sediment content and a height greater than 5mm. This type of benthic community is not conducive to the settlement of coral larvae and is not consumed by herbivorous fish. The morphological characteristics and organic matter content of EAM exhibit obvious differences in substrate, depth, and seasons. Furthermore, the rich organic matter in the EAM can provide potential food resources for numerous benthic small invertebrates, which play an essential and significant role in the food web of the coral reef ecosystem.

Key words: Epilithic algal matrix, coverage, morphological characteristics, spatiotemporal differences, Sanya bay