SCTLD has been described by scientists as the largest and most severe coral disease outbreak ever documented. It affects more than twenty stony coral species, and its spread along the Florida reef tract followed a pattern that strongly suggested ocean currents were the primary courier: an initial exponential expansion outward from the disease epicenter near Miami, followed by a persistent, linear march northward along the Southeast Florida reefs and southward into the Florida Keys. Earlier modeling work by coauthor Thomas Dobbelaere and colleagues, using a coupled hydrodynamic-epidemiological framework, confirmed that depth-averaged currents explained the progression of the disease front well and estimated that the pathogen likely persists in the water with a half-life of approximately thirty days. That long waterborne lifetime is central to the new findings, because it means disease particles remain infectious far longer than most coral larvae remain competent to settle.

To build their dual dispersal picture, the team employed SLIM, an unstructured-mesh, depth-integrated coastal ocean model covering Florida’s Coral Reef, the Florida Strait, and the eastern Gulf of Mexico over the period from May 2018 to January 2021. The computational mesh contained roughly 700,000 elements, with resolution refined to about 100 meters along the reef tract and coarsening to around 10 kilometers offshore, allowing the model to resolve fine-scale features such as recirculating eddies and channelized currents between individual reefs and islands. Deeper, shelf-break mesoscale circulation was relaxed toward fields from the HYbrid Coordinate Ocean Model. Into this hydrodynamic engine, the researchers released virtual particles representing both coral larvae and SCTLD propagules, with particle motion governed by advection-diffusion dynamics solved with a fourth-order Runge-Kutta scheme and a stochastic diffusivity term estimated from classical oceanic diffusion diagrams.

The biological realism of the larval simulations rested on species-specific spawning schedules. Five species were modeled: staghorn coral Acropora cervicornis, mountainous star coral Orbicella faveolata, pillar coral Dendrogyra cylindrus, symmetrical brain coral Colpophyllia natans, and symmetric brain coral Pseudodiploria strigosa. Roughly one million virtual larvae per species were released per spawning event, timed according to species-specific lunar and daily periodicity in late summer. After a precompetency period of 5.12 days, larvae acquired competency to settle at a rate of 0.063 per day and lost competency at 0.016 per day, while mortality followed a Weibull survival model. The SCTLD agent, by contrast, was modeled as a neutrally buoyant tracer whose infectivity decayed exponentially with the thirty-day half-life inferred from earlier epidemiological fitting. The consequences of this difference proved profound: because infectious particles survive far longer than larval competence windows, they sample a much wider range of oceanographic conditions, including current reversals and eddies that carry them in essentially all directions across the reef.

The results, summarized in enormous connectivity matrices built from about 14,000 reef cells and nearly 200 million potential connections, delivered a stark headline finding: during the disease outbreak, populations on Florida’s Coral Reef were more connected by disease than by coral larvae. The disease agent formed a single strongly connected component spanning almost the entire reef tract, meaning virtually every reef can be reached by the pathogen, even if only through multi-step, low-probability pathways. Coral larvae, by contrast, formed species-specific networks with distinct strongly connected neighborhoods in the Upper, Middle, and Lower Keys and the Dry Tortugas, a separate component in Southeast Florida, and highly asymmetric, northward-skewed connectivity in the northern reef tract where the powerful Florida Current dominates. Connection strength ranked highest for A. cervicornis, followed by C. natans, P. strigosa, O. faveolata, and finally D. cylindrus, whose restricted spawning window yielded the most fragmented network.

The analytical core of the study is a new composite restoration index, R, that weighs a reef’s value as a larval source against its exposure to incoming disease. The index penalizes a reef’s normalized weighted coral out-degree, a measure of downstream seeding potential, by its normalized weighted disease in-degree, a measure of incoming pathogen connectivity, with a tunable parameter beta controlling the relative importance of disease risk. Scores range from minus one to one, with negative values indicating locations where disease exposure overwhelms any seeding benefit. By taking the median score across the range of beta values and applying spatial clustering, the team identified disease-penalized restoration hotspots throughout the reef tract, offering managers a tunable, species-specific planning tool rather than a single rigid ranking.

The spatial patterns that emerged were striking and, in some cases, counterintuitive. Reefs in Southeast Florida, despite strong larval export potential, received highly variable and often low restoration scores because they sit downstream of diverse and strong incoming disease connections from the outbreak’s original epicenter. Nearby reefs in Biscayne Bay and the northernmost Upper Keys, by contrast, maintained consistently positive scores even under heavy disease weighting, thanks to comparatively reduced incoming pathogen exposure. Broad hotspots appeared across Biscayne Bay, the northern Upper Keys, the Middle-Upper Keys transition, and throughout the Lower Keys, including a multispecies hotspot stretching from Sugarloaf Key to Key West. Species with similar spawning windows, such as A. cervicornis and O. faveolata, and separately C. natans and P. strigosa, produced closely matching hotspot maps, suggesting that paired, multispecies outplanting strategies could be designed around shared geography.

The study also scored the seven flagship sites of NOAA’s Mission: Iconic Reefs program, an ambitious federal initiative to restore some of the most historically significant reefs in Florida. Cheeca Rocks and Carysfort reefs emerged as exceptional multispecies candidates, combining strong seeding potential with comparatively low predicted disease exposure across most scenarios, while Sombrero, Looe, and Dry Rocks reefs scored consistently poorly, likely reflecting high disease exposure even though some of these sites may still function as quality larval sources. Newfound Harbour showed mixed results depending on the species. The authors stress that these scores represent one layer of information in a multifactorial decision landscape that also includes habitat suitability, historical significance, logistics, and the widely varying susceptibility of different coral species to SCTLD; staghorn coral, for example, is expected to be relatively resistant, while pillar coral is highly vulnerable.

For reef managers confronting accelerating environmental change, the study reframes a foundational assumption of restoration ecology. Population connectivity has long been treated as an unqualified good, the mechanism by which outplanted colonies rescue degraded reefs by replenishing them with larvae across generations. The new work demonstrates that in an era of emergent disease, connectivity is a double-edged sword: the same currents and sub-mesoscale oceanographic features that knit coral populations together also knit their epidemics together. The researchers acknowledge important limitations, including the assumption of similar larval traits across species, the exclusion of three-dimensional flows and wave-driven processes, and a model period limited to about four years, which cannot fully capture the influence of stochastic events such as hurricanes, which earlier work has shown can both enhance larval connectivity and supercharge disease spread.

Nevertheless, the practical implications are already flowing into policy. The team has summarized its data and provided it to the Florida Department of Environmental Protection for use in natural resource management planning, and the open-source nature of the restoration index means it can be adapted to other coastal ecosystems facing similar disease threats. As ocean warming and global trade continue to fuel the emergence of novel pathogens in coastal seas, the study’s central message is likely to resonate far beyond Florida: restoration planning that ignores disease connectivity is planning with half the map. The reefs best positioned to seed the future, the simulations suggest, are not simply the best-connected ones, but those where the currents deliver larvae without delivering the plague.

Subject of Research: Balancing larval dispersal benefits and disease transmission risks of population connectivity for coral restoration on Florida's Coral Reef

Article Title: Balancing the benefits and risks of population connectivity to coral restoration in the age of emergent disease

Article References: Holstein, D. M., Dobbelaere, T., Gramer, L. J., McEachron, L., Muller, E. M., Williams, S. D., & Hanert, E. (2026). Balancing the benefits and risks of population connectivity to coral restoration in the age of emergent disease. Coral Reefs. https://doi.org/10.1007/s00338-026-02951-7

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02951-7

Keywords: coral reefs, stony coral tissue loss disease, coral restoration, population connectivity, larval dispersal, biophysical modeling, Florida, ocean currents, emergent disease, Mission Iconic Reefs, hydrodynamic simulation, marine conservation