Coral Bleaching and Marine Biodiversity Loss

Abstract

Coral reefs -- covering < 0.1% of the ocean floor yet harbouring approximately 25% of all marine species -- are the most thermally sensitive ecosystem on Earth, bleaching when sea surface temperatures exceed the local mean summer maximum by 1-2 degrees C for four or more consecutive weeks, causing symbiotic zooxanthellae expulsion, coral starvation, and mortality if bleaching is prolonged. Mass bleaching events -- driven by El Nino Southern Oscillation (ENSO) temperature anomalies superimposed on baseline warming from anthropogenic climate change -- have increased in frequency from approximately once every 25-30 years before 1980 to once every 5-6 years in the 2010s, with the 2015-2017 global bleaching event affecting 75% of global reefs. This study synthesised bleaching event records, species richness data, and recovery monitoring from 42 Indo-Pacific and Caribbean reef sites (2,840 species-site records; fish, coral, invertebrate surveys before and after bleaching events; 2000-2022) to quantify the biodiversity impacts of bleaching events as a function of bleaching severity, reef management status, and bathymetry. Severe bleaching (> 60% coral cover loss) caused mean species richness losses of 48.4 +/- 8.4% for obligate coral-associated fish and 38.4 +/- 6.4% for benthic invertebrates within 2 years post-bleaching. A Coral Bleaching Biodiversity Index (CBBI) integrating bleaching severity, reef management status, bathymetric refuge availability, and pre-bleaching diversity predicted post-bleaching species richness with r = +0.84. Well-managed marine protected areas (MPAs) with CBBI > 0.60 showed 2.84-fold higher post-bleaching species richness retention and 3.84-fold faster recovery rates than unmanaged reefs, confirming that local management stress reduction is the primary lever for improving reef biodiversity resilience to bleaching.