Stony coral tissue loss disease decimated Caribbean coral populations and reshaped reef functionality

Coral reef health is severely threatened by diseases, which cause significant declines in coral abundance and reef functionality. A major outbreak of stony coral tissue loss disease (SCTLD) began in Florida in 2014 and rapidly spread throughout the Caribbean, impacting a wide range of coral species. Previous disease outbreaks, such as white-band disease in the late 1970s, already devastated Acropora populations, highlighting the vulnerability of Caribbean reefs. The current SCTLD outbreak is particularly concerning due to its rapid spread and high mortality rates. The precise cause of SCTLD remains unclear, though evidence suggests a complex interaction of environmental factors and pathogens. This study investigated the regional effects of SCTLD on coral communities in the Mexican Caribbean, analyzing the impact on coral populations, functional diversity, and reef calcification. Understanding the consequences of SCTLD is crucial for predicting future reef conditions and developing effective conservation strategies.

The Caribbean has a history of devastating coral disease outbreaks. White-band disease in the late 1970s caused significant declines in Acropora palmata and A. cervicornis, drastically reducing reef functionality. Subsequent disease outbreaks further decimated key reef-building corals. The increasing frequency and intensity of these events are linked to human-induced pressures, such as rising ocean temperatures, degraded water quality, and nutrient enrichment. SCTLD, first observed in 2014, represents a significant new threat, affecting nearly 30 coral species with high virulence and rapid spread. Studies indicate a possible multifactorial etiology involving environmental conditions and pathogen-induced dysbiosis. While the origin was Florida and Mexico, similar impacts are seen elsewhere. The study builds upon existing knowledge of coral disease dynamics in the Caribbean, providing a detailed assessment of the SCTLD outbreak's ecological consequences.

This study used extensive pre- and post-SCTLD data collected along a 450-km reef track in the Mexican Caribbean between July 2018 and January 2020. A total of 29,095 colonies were surveyed across 101 reef sites. Thirty-five sites were also surveyed in 2016 (pre-outbreak). Surveys recorded coral species, colony size, disease prevalence, and mortality. Six functional traits (skeletal density, growth rate, rugosity index, colony size, reproduction strategy, and corallite width) were used to characterize functional diversity. The impact of environmental and anthropogenic covariates (depth, reef zone, structural complexity, coral density, coastal development, wind exposure, and MPA age) on SCTLD prevalence was analyzed using logistic mixed models. Changes in coral community composition, functional diversity (richness and evenness), and calcification were assessed using principal component analysis (PCA), SIMPER analysis, and paired t-tests. Coral community calcification was calculated using colony size, growth rate, skeletal density, and morphology. Statistical analyses were performed in R.

SCTLD rapidly spread across hundreds of kilometers within months, causing unprecedented coral loss. 17% of surveyed colonies were dead with recent outbreak signs, with an additional 10% affected. Mortality varied widely among species; Meandrinidae and Faviinae families were most severely impacted. Disease prevalence was not significantly related to depth, reef zone, structural complexity, or coral density, suggesting water column pathogen transport as a primary driver. However, coastal development, wind exposure (inward areas), and MPA age were strongly associated with higher disease prevalence. The isolated Banco Chinchorro reef remained unaffected, possibly due to its remoteness and limited human impact. The loss of highly susceptible species led to a significant reduction in functional diversity and evenness and a substantial decrease in coral community calcification (nearly 30%). The post-outbreak communities were dominated by opportunistic corals with simpler morphologies and slower growth rates, although acroporid corals showed an apparent relative increase, likely due to the decline of other species.

The findings demonstrate the catastrophic impact of SCTLD on Caribbean coral reefs, leading to significant shifts in community structure and function. The rapid spread of the disease highlights the importance of waterborne pathogen transport. The strong association between coastal development and disease prevalence emphasizes the role of anthropogenic pressures in increasing reef vulnerability. The reduced functional diversity and calcification rates suggest a long-term impact on reef resilience and ecosystem services. While acroporids show a relative increase, their overall abundance remains low, limiting their capacity to restore reef structure and function. The loss of functionally important corals compromises vital processes like reef framework production and habitat complexity maintenance.

The SCTLD outbreak is a major threat to Caribbean coral reefs, causing dramatic losses in coral populations, functional diversity, and calcification rates. Anthropogenic pressures exacerbate reef vulnerability. While some opportunistic species may increase, the overall loss of key functional groups jeopardizes reef health and ecosystem services. Future research should focus on understanding the disease's etiology, developing effective mitigation strategies, and promoting coral recovery through interventions like active restoration and habitat management.

The study focuses on a single region in the Mexican Caribbean; the results may not be fully generalizable to other areas. The study relies on observational data, and causal relationships between covariates and disease prevalence can only be inferred. The assessment of coral mortality attributed to SCTLD relies on visual observations, which might underestimate the true extent of the disease's impact.