Dynamics of coral community structure after the 2016 mass bleaching event along the Western Coast of Sumatra, Indonesia

Global climate change has led to an increasing seawater temperature, which in turn triggers widespread coral bleaching events. Similar impacts have been observed in Indonesian waters, especially along the western coastal region of Sumatra. A time-series study has been conducted across a wide regional system encompassing North Nias, Central Tapanuli, Mentawai, and Pieh. Surveys were undertaken over three periods: 2015 (pre-bleaching), 2016 (during bleaching), and 2018/2019 (post-bleaching). A network of 39 monitoring stations established with permanent transect provided a strong basis for assessing both spatial and temporal change across the region. The Underwater Photo Transect (UPT) method was used along these permanent transects, with a total of 1950 photographs for analysis. The analysis revealed a substantial decline in live coral cover (HC), from 37.3% ± 19.1% before bleaching to 19.4% ± 14.6% during the bleaching event. In the post-bleaching period, there was an increase reaching to 22% ± 18.9%. A similar decline trend was also observed in coral community structure, reflected by a reduction in the Shannon diversity index from 1.84 to 1.43 during bleaching, followed by a further decline to 1.31 in the post-bleaching phase. This pattern indicates a significant decrease in coral genus diversity associated with thermal stress. The decline in coral genera has caused a shift in morphological composition, resulting in a decrease in the reef's structural complexity. Elevated sea temperatures altered the distribution of coral genera, which in turn influenced reef morphological complexity and potentially broader ecological functions. The genera Porites, Acropora, and Montipora were the most dominant genera across all sites. However, a change in composition occurred, characterized by a decline in Acropora and being replaced by Pavona, indicating that more thermally tolerant taxa became increasingly dominant relative to more sensitive groups. After three years, an initial recovery was seen with an increase in the number of colonies. Nevertheless, coral communities had not yet returned to pre-bleaching conditions. These findings demonstrate that rising temperatures have reshaped coral genus configurations, reduced morphological complexity, and altered reef ecosystem functioning. Increasing the pressure due to climate changes, require integrated management strategies to reduce anthropogenic stressors, to enhance reef resilience to support sustainable long-term coral reef ecosystem services.