Variations in hard coral and Macroalgal Cover on Pareiwella Reef, (Tangalle, Sri Lanka) in response to the 2023 El Niño and Post-Event Recovery

Coral reefs, essential for marine biodiversity and ecosystem services, are increasingly threatened by climate anomalies such as El Niño. Sri Lanka has also been affected by El Niño events over the past few decades. This study provides the first documented assessment of coral bleaching impacts and short-term recovery dynamics on a Sri Lankan reef system. We evaluated variation in Hard Coral Cover (HCC), Macroalgal Cover (MaC), and Other Benthic Cover (OBC) in the back reef zone of the Pareiwella Coral Reef, Tangalle, Sri Lanka, during the El Niño event (August–October 2023) and the post-El Niño period (January–March 2025). Bleaching assessments for HCC were conducted in August 2023 and January 2025, classifying colonies as totally bleached (TBC), partially bleached (PBC), pale-colored (PCC), recently dead (RDC), dead (DC), or healthy (HC). Data were collected along six permanent transects with 0.5 × 0.5 m² quadrats deployed at regular intervals. During the El Niño period, mean percentage cover± SD of HCC, MaC, and OBC was 35.77 ± 1.97%, 47.39 13 ± 2.80%, and 16.84 ± 2.36%, respectively. HCC and OBC of post-El Niño values increased to 56.61 ± 5.63%, 18.23 ± 3.95%, while MaC decreased to 40.87 ± 4.89%. Bleaching severity during El Niño showed high TBC (11.31%) and PCC (16.93%), and only 42.39% healthy colonies, whereas post-El Niño assessments indicated an increase in healthy colonies to 53.26 ± 8.92%. Repeated measures ANOVA revealed significant variation in HCC among coral genera (p < 0.05), with notable genus time interactions. Monthly variation in HCC was only significant in the post-El Niño period. MaC showed no significant temporal or taxon-level differences, although Caulerpa spp. remained consistently dominant. The decline in HCC during El Niño was linked to repeated bleaching impacts, especially in non-dominant genera. In contrast, partial recovery in resilient genera following the event corresponded with improved environmental conditions. These findings highlight taxon-specific benthic responses to the 2023 El Niño event and provide insight into post-event coral recovery, with direct implications for reef monitoring and management.