El Niño-driven phase shift to algal dominance on Isla del Caño's coral reefs: implications for urgent restoration

The 2023-24 El Niño event caused extreme marine heat stress and widespread coral bleaching. Coral reefs at the Reserva Biológica Isla del Caño and the northern coast of the Osa Peninsula, Costa Rica, underpin critical ecosystem services, including biodiversity conservation and marine tourism, and have previously withstood similar events with minimal coral loss. Evaluating the ecological impacts of the 2023-24 El Niño is essential to assess coral reef resilience and guide future management.

Coral Reef Watch sea surface temperature (SST) data (1985-2025; CoralTemp V3.1) were used to calculate long-term SST trends and degree heating weeks (DHW). Reef surveys were conducted at nine sites between 2019 and 2025, with primary benthic composition and coral health data collected in 2024-25. Benthic cover was assessed using point-intercept, line-intercept, and quadrat methods, while coral diversity, abundance, and health were measured via belt transects. Beta regression was used to assess the effect of temperature on coral cover, and multivariate analyses, including principal component analysis (PCA) and similarity percentage analysis (SIMPER), evaluated benthic community changes and bleaching patterns. An Ecological Recovery Feasibility Index (ERFI) was developed using PCA loadings and benthic indicators to rank sites by recovery potential.

SST increased significantly over the past 40 years (∼0.23 °C/decade), with the 2023-24 El Niño recording peak SST (31.2 °C). Bleaching threshold exceedance days increased, while cool days declined. Twelve coral taxa were recorded; Pocillopora spp. and Porites lobata were present at all sites. Coral diversity varied, with Cueva and Ancla highest, and San Josecito lowest. Estimated baseline bleaching prevalence was ∼23%, highest in Pocillopora spp. (33.9%). SIMPER and PCA revealed a shift from coral to algal dominance: turf algae increased by 70.62%, dead coral declined 80.71%, and coral cover fell 40.44%. Major coral declines were statistically significant at Ancla, Esquina, and Tina. Bayesian regression confirmed coral decline at Chorro, Cueva, Tina, and Ancla, alongside turf algae increases. Coral cover was higher at warmer sites, though non-temperature site-specific factors were also influential. Chorro and Esquina had the highest recovery potential; Ancla, San Josecito, and Barco Profundo the lowest.

There is an urgent need to develop and implement a coral reef restoration strategy for Isla del Caño that addresses site-specific conditions, integrates tourism management, and promotes long-term resilience. Under continued climate change, localized, targeted restoration will be essential to maintain the ecological function of these historically resilient but increasingly vulnerable reefs in Costa Rica's Eastern Tropical Pacific.