Bleaching Events and Their Long-Term Effects on Coral Reef Fish Communities: Mechanisms, Trophic Cascades, and Implications Under Accelerating Climate Change

Abstract

Coral bleaching, the thermally and irradiance-driven expulsion of endosymbiotic Symbiodiniaceae from the gastrodermal cells of scleractinian corals, has emerged as the most ecologically devastating manifestation of anthropogenic climate change on marine ecosystems. Since the first documented mass bleaching event in 1982–83, the frequency, spatial extent, and thermal severity of bleaching episodes have escalated dramatically, culminating in the second and third global bleaching events of 2010 and 2014–2017, during which degree heating week (DHW) accumulations of unprecedented magnitude drove coral mortality on scales never previously recorded. Coral reef fish communities — which encompass an estimated 25% of all known marine fish species within an ecosystem covering less than 0.1% of the global ocean floor — are not merely passive witnesses to coral degradation but active participants in a complex web of structural, trophic, and chemical ecological dependencies that are systematically dismantled as bleaching transforms coral-dominated reef benthos into algal-dominated rubble fields. This article provides a comprehensive and mechanistically detailed analysis of bleaching event dynamics and their cascading short-term and long-term consequences for reef fish communities, integrating evidence from cellular coral physiology, fish behavioural ecology, community-level field surveys, stable isotope trophic analyses, and climate projection modelling. We examine the molecular cascade initiated by thermal stress in the Symbiodiniaceae–coral symbiosis — from PSII photoinhibition and reactive oxygen species overflow to caspase-3-mediated apoptosis and host tissue necrosis — as the mechanistic foundation for understanding why and how rapidly structural and trophic resources available to fish deteriorate following bleaching. The differential vulnerability of reef fish functional groups — from obligate coral-dwelling gobies and corallivorous butterflyfishes to herbivorous parrotfishes, planktivorous damselfishes, and apex predatory groupers — is analyzed in terms of the specific habitat, food, and refuge dependencies that link each guild to living coral in distinct and quantifiable ways. Long-term community trajectory data from the Great Barrier Reef, Caribbean, Indian Ocean, and Pacific island reefs document the progression from species-rich, trophically complex pre-bleaching communities to depauperate, low-complexity post-bleaching assemblages dominated by habitat generalists, with trophic diversity index (H') declines of 35–45% and structural complexity index reductions of 55–75% documented over decade-scale post-bleaching monitoring. The critical mediating roles of herbivore functional group integrity, post-bleaching substrate trajectory (coral recovery vs. algal phase shift), and local stressor interactions in determining fish community recovery potential are evaluated. Finally, projected reef fish community outcomes under the four major IPCC Shared Socioeconomic Pathway scenarios are synthesized, revealing that only aggressive near-term emissions mitigation consistent with SSP1-1.9 offers a realistic pathway to preserving functionally viable reef fish communities beyond mid-century.