The effects of Stony Coral Tissue Loss Disease on TCI reefs.

By Heidi Hertler, John Debuysser, Autumn Zwiernik, Katie Tanner, Alyssa Landi, Hayley Newman and Morgan Rose, The School for Field Studies Center for Marine Resource Studies, South Caicos

Coral reefs are one of the most biologically diverse and ecologically productive ecosystems on the planet. They are aesthetically attractive and economically valuable. These distinct ecosystems provide essential ecological, social, and economic value to millions of people globally. Not only do coral reefs provide habitat essential for nearshore fisheries, they also support tourism and protect coastlines from natural disasters. 

In 2019, researchers at The School for Field Studies in South Caicos (SFS) noticed a change in the local reefs. Suddenly, at multiple sites, corals were dying; sometimes just one area on an individual coral and other times entire coral heads. It was quickly determined that a disease was causing this sudden death of previously healthy corals. Coral diseases have been studied for years, and in many cases, researchers have been able to identify the causes, transmission factors, and solutions to such diseases. First reported in Florida in 2014, Stony Coral Tissue Loss Disease (SCTLD) was killing corals faster and across more species than any other coral disease known. Over half of the stony coral species found on the Florida Reef Tract are affected by SCTLD, though there are many factors that influence the likelihood that a coral will contract the disease, such as the location of the coral and time of the year. Many researchers throughout Florida and the Caribbean are actively trying to pinpoint the indicators of the disease and the rate at which it progresses, though it has proven to be a difficult feat as these attributes may vary by species affected. SCTLD has largely moved through the Turks & Caicos Islands.

Studies of the South Caicos reefs have varied over the years but largely revolve around the topic of reef health. Coral recruitment — the settlement of coral larvae on suitable substrate — is a key process for recovery and resiliency of reef-building corals. Recruitment is sensitive to a number of factors including live coral cover, larval abundance, hydrodynamics, sedimentation, algal competition, and site connectivity, producing large spatio-temporal variations in recruitment.

These images of coral at the Lost Anchor dive site were taken after the ravages of SCTLD, suggesting some resilience or rebound from its effects.

In 2015, John DeBuysser, a former SFS student and current SFS dive safety officer, surveyed coral recruits in addition to the routine benthic assessment. The greatest live coral cover was found at the Maze (8.2%, 20 m depth) followed by the Arch (7.6%, 10 meters depth) and the lowest at the Plane (0.92%, 10 meters depth). The reefs surrounding South Caicos are dominated by lettuce coral (Agaricia agaricites), lobed star coral (Orbicella annularis), boulder star coral (Orbicella franksi) and massive starlet coral (Siderastrea siderea). Of course, not all reefs are created the same. For example, East Caicos reefs are characterized by mountainous star coral (Orbicella faveolata), lobed star coral (Orbicella annularis), boulder star coral (Orbicella franksi) and mustard hill coral (Porites astreoides).

Just like not all reef systems are the same, not all coral use the same reproductive strategy — there are broadcasters and brooders. This generally occurs annually during mass spawning events in one or two consecutive months and trigged by water temperature, lunar and tidal cycle, and light. Many corals are typically hermaphroditic, both sexes are represented in each coal polyp. Broadcaster coral release sperm and eggs into the water column. A planula larva is created when egg and sperm connect. In the Caribbean region, large reef-building corals, such as Acropora, Montastraea, Orbicella, and Siderastrea, are generally broadcast spawners. The pelagic larvae of these genera are more susceptible to mortality and recruitment to reef habitat is dependent on prevailing current patterns of the area.

It has been noted that these reef-building corals typically have much lower abundance in recruitment studies than brooding coral genera, Agaricia and Porites, who develop their planula larvae internally or on the surface of the parent colony before releasing. Brooding corals tend to be smaller in size. Once the larvae settle, asexual reproduction begins and the colony starts to grow.

The overall average coral recruitment density in 2016 of all sites was 15.9 ± 17.9 recruits/m² with the highest number at the Arch (29.6 ± 25.5 recruits/m², 10 meters depth) and the lowest at the Chain (8.3 ± 12.3 recruits/m², 20 meters depth). This is greater or comparable to other areas in around the Caribbean.

The predominant genera in 2016 of the recruit community was Siderastrea, a broadcast spawner. Addtionally, both recruit and mature coral communities were characterized by presence of Agaricia, Diploria, Montastraea, Orbicella, Porites, and Siderastrea. These findings suggest strong connectivity in the area since the sites are able to recruit both pelagic broadcast-spawning coral larvae and local brooding coral larvae. From recruit density and diversity it was evident that recruitment in this area was able to successfully maintain coral community structure.

In addition to recruit and fish surveys, SFS students conduct regular benthic surveys to help understand reef dynamics.

Prior to 2019 onset of SCTLD, the reefs of South Caicos remained relatively stable. Stony coral cover (3.0%) and richness (9.9 taxa) did not fluctuate significantly (2012–2018). Disease monitoring efforts show that the regional outbreak of SCTLD poses a serious threat to important framework-building corals throughout the Caribbean. In TCI, SCTLD significantly impacted coral cover (1.5%) and richness (6.3 taxa) (Figure 1).  South Caicos reefs are now dominated by massive starlet coral (Siderastrea siderea), lettuce coral (Agaricia agaricites), and mustard hill coral (Porites astreoides). Reef core evidence and modern surveys show that weedy or stress-tolerant taxa such as Siderastrea spp. and Porites spp., which used to be rare in reef assemblages, are now prevalent in the Caribbean.

The overall average coral recruitment density in 2022 for all sites was 9.34 ± 8.99 recruits/m², not significantly different from 2016; however, the most abundant species in terms of count of recruits by coral surveyors are now Siderastrea sidereal and Porites porites (Figure 1). Rare and susceptible taxa corals including elliptical star coral (Dichocoenia stokesii), smooth flower coral (Eusmilia fastigiate), grooved brain coral (Diploria labyrinthiformis) and maze coral (Meandrina meandrites) were absent from all surveys for the first time; however they were still observed on the reef (Photos from April 2022).

Although recruit density was different among sites compared to 2016, there were no correlations between recruitment density and coverage of key substrate types — live coral, coralline algae, macroalgae, and pavement.  These findings suggest that another factor is at play influencing the recruitment patterns. 

Ecological shifts associated with SCTLD in South Caicos are reported in the Turks & Caicos Islands and support other studies in the Caribbean which demonstrate that SCTLD significantly deteriorates reef composition. Coral reefs have a complex symbiotic relationship with reef fish, providing shelter, nursery grounds, and food. In turn, fish help keep coral reefs algae-free which promotes reproduction and growth.

Previous studies have concluded that fish species decrease in number after coral cover has declined from disturbances such as disease and climate change. Fish surveys are included in SFS routine reef assessment. The Admiral Cockburn Land and Sea National Park saw a 20% increase in number of species, mostly invertivores; in addition, there was a shift in dominant species based on class. In 2016, carnivores were dominated by schoolmaster snappers whereas in 2022 this class was dominated by groupers. A healthy population of parrot fish dominate the herbivore group in both 2016 and 2022.  

What is next for TCI reefs? Coral reefs are keystone ecosystems vital to ocean health; however, the number and prevalence of diseases in the Caribbean affecting corals is increasing dramatically starting in the 1990s, and are recognized as a major factor in reef decline since the early 2000s. Changing climate will continue to threaten our reefs.

We must be part of the next step and prepare for the future. Qualitative and quantitative research allow us to assess impacts and develop opportunities for reef conservation. Marine Protected Area are vital for promoting healthy reefs but may not be enough in these changing times. Restoration efforts can play a significant role. Partnership between NGO, research organizations, and business can make this happen. We must be prepared for what the future brings and help our reefs weather the changing environment.