Disease and bleaching drive divergent net carbonate production across Caribbean reef systems

Over the past decades, persistent environmental degradation has compromised the ability of many tropical reefs to maintain positive carbonate budgets-fundamental indicators of reef growth potential and critical to maintaining structural complexity and ecosystem function.¹ This chronic deterioration is exacerbated by extreme climate-induced disturbances, which can abruptly tip the balance from net carbonate production to net erosion at regional to global scales. The 2023 thermal stress event caused the most extensive and severe global reef stress on record.² Here, we assess its impact on carbonate budgets across 68 reef sites in the Caribbean and Gulf of Mexico (GoM). Notably, the Caribbean sites were also affected by the recent stony coral tissue loss disease (SCTLD) outbreak,³ providing a unique opportunity to differentiate thermal stress impacts both individually (GoM) and in combination with SCTLD (Caribbean). We show that 70%-75% of Caribbean reef sites shifted to net-erosional states following the heat stress, driven by widespread loss of branching and plate-forming corals. These impacts compounded previous SCTLD effects, which had already significantly reduced large and massive taxa. By contrast, GoM reefs, unaffected by SCTLD, largely retained positive carbonate budgets due to the dominance of massive reef builders that better resisted heat-related mortality. These divergent responses highlight how differences in community composition, even between adjacent biogeographic regions, determine the vulnerability of carbonate production to acute thermal stress events and underscore the urgent need for targeted management and global climate mitigation to preserve reef ecosystems.