El Niño Heat Drives Algal Takeover in Costa Rica's Resilient Reefs

The marine heatwave triggered by the 2023–24 El Niño event has dealt a severe blow to the coral reefs of Isla del Caño in Costa Rica, an ecosystem previously celebrated for its ability to withstand thermal stress. New research indicates that extreme sea surface temperatures, peaking at 31.2°C, have catalysed a worrying phase shift: the reefs are transitioning from being dominated by vibrant coral to being overrun by algae.

Long-term analysis reveals that local sea surface temperatures have risen by approximately 0.23°C per decade since 1985. This warming trend culminated recently in widespread stress, with survey results showing a 40.44% drop in coral cover and a staggering 70.62% increase in turf algae—simple, fast-growing marine plants that can rapidly colonise open space. Pocillopora, a branching coral genus crucial to the reef structure, suffered the highest prevalence of bleaching.

Interestingly, the study found that coral cover remained higher at some warmer sites, suggesting that complex local factors beyond just temperature influence survival. However, with the algae seizing territory from dead coral, natural recovery is becoming increasingly difficult. The researchers argue that passive conservation is no longer enough. To save these vital underwater structures, which underpin biodiversity and tourism, an urgent strategy of active, localised restoration is now essential to bolster their resilience against future climate shocks.