When Life Depends on Corals, What Marks the Most Promising Reef?

January 15, 2014

Corals that survived the bleaching (S. siderea at the bottom left and Diploria labyrinthiformis at the top right) continue to offer food and homes for economically important fish and shoreline protection. Credit: Jonathan Gomez.

The Study: Jahson Berhane Alemu I, Ysharda Clement. 2014. Mass Coral Bleaching in 2010 in the Southern Caribbean. PLOS ONE January 06, 2014. DOI: 10.1371/journal.pone.0083829

The Big Question: On the Caribbean island of Tobago, more than half the economy depends on corals—for tourism, fisheries and shoreline protection.  Coral bleaching–caused by episodes of unusually warm ocean water–is a major threat, potentially exacerbated by freshwater runoff and other stressors. So how can islands like Tobago cope with that threat?

By studying the response of dozens of coral species to a mass bleaching event in the southern Caribbean in 2010, researchers at the Trinidad and Tobago Institute of Marine Affairs aimed to identify species and locations that were less affected by bleaching and might particularly benefit from protection.

“Given limited resources and local capacity,” says lead author Jahson Berhane Alemu I., “we are basically trying to find and protect the most resilient reefs. The hope is that this study adds a new criterion for the selection of marine protected area sites.” (Alemu I. participated in a Nature Conservancy science writing workshop to refine the paper.)

Study Nuts and Bolts: The team tracked coral bleaching, coral die-off and algal overgrowth at sites on three of the largest coral reef systems surrounding Tobago. One site (Buccoo) is an existing park, but protections are modest. Another (Speyside) is being proposed as a marine protected area. At each site, they measured the size and estimated the percent bleaching for every colony over 5 cm that they found in four (10 x 2 m) transects.

They monitored water temperature hourly and collected bleaching and die-off data monthly from October 2010-March 2011–a period that began with the highest two months of sea-surface temperatures recorded in the region during the past decade.

They sampled bottom cover before the bleaching event and again at one and two years after the event.

The Findings:  The researchers observed about 650 colonies in 30 coral taxa. At the beginning of the study, hard corals covered 17-26% of the seafloor. By the end of the study, the sites had each lost between about 35% and 45% of the corals cover, though — surprisingly — the mix of species at each site remained about the same.

Differences among Species

The researchers focused on four reef-building taxa — species that lay down the structure for a greater diversity of corals and fish. These include the great star coral (Montastraea cavernosa) the mountainous star coral (Montastraea faveolata), massive starlet coral (Siderastrea siderea) and the boulder brain coral (Colpophyllia natans).

These reef building corals took a heavy hit in an earlier (2005) bleaching event, experiencing a 75% die off. The 2010 event was a double-whammy for the particularly vulnerable C. natans; colonies saw lower bleaching than most other taxa, but higher mortality (5-8%).

This study found as others have that the great star coral (M. cavernosa) is fairly resilient. At one site (Speyside), over 80% of the colonies bleached, but very few died.

In some cases such as the encrusting corals Mycetophyllia aliciae and Mycetophyllia ferrox —they found little bleaching but rather sudden and almost complete die-off. “Over a period of 7 days, says Alemu I, “I was able to observe Mycetophyllia colonies blacken and die off…to the delight of fireworms.” (Fireworms primarily eat dead things and are well-adapted to devouring corals.  They suck the polyps right out of the coral skeletons.)

Record high water temperatures in 2010 contributed to severe bleaching in this Siderastraea siderea (starlet coral) specimen at Speyside reef off the coast of Tobago. Ten days after this photo was taken at the end of September 2010, all the gorgonians and sponges in the background had died. Credit: Jonathan Gomez.

Differences among Sites

For locating marine protected areas and for setting management priorities differences between sites matter as much as differences between species, the researchers found.

While an increase in water temperatures mean more frequent and severe bleaching events are inevitable, it’s important to understand how temperatures interact with other factors. Freshwater (often turbid) runoff from land can cut two ways. The turbidity can reduce penetration of ultraviolet light and help protect corals from that added stress. Or the fresh water itself can exacerbate heat-related bleaching.

A lens of fresh water sat over all three sites during the bleaching event, but it was deepest at Speyside and seemed to contribute to greater bleaching there most dramatically in star corals (M. cavernosa) which showed greater than 80% bleaching at Speyside, v. less than 5% at other sites.



Mean surface temp during study

Days above

29 °C

Depth of freshwater lens



27.7 °C


3-5 m



27.6 °C


3-5 m



28.5 °C


9 m


Unlike many Caribbean islands, Tobago is subject to influences from South America including nutrient-rich runoff on a year round basis. This South American influence contributes to the problem and is beyond local control — but the study also underscores the importance of local watershed management.

What It All Means         

This study gathered extensive data on several key sites during a period of extreme stress. The authors could not attribute every result to a single cause, but began to get a handle on which influences may be most important on these reefs.

Additional studies are underway to examine related factors in greater detail including water quality impacts, fish distributions, and even genetic connectivity among sites. As managers and government officials consider whether and where to locate a proposed marine protected area, they are also exploring the state of coastal development, waste management and watershed use.

“Corals respond differently to the many stresses they face,” says Stephanie Wear, the Nature Conservancy’s lead scientist for coral reef conservation. “This study highlights how important it is to understand these variances locally so that reef managers can tailor their responses to the reefs they manage.”

Goods and services associated with coral reefs including tourism, fisheries and shoreline protection contribute up to $160M to the Tobago economy. Even an incremental improvement in practices that protect reefs could offer greater economic benefits.C

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1 comment

  1. This study represents a major stepping stone in the development of marine protected area policy in T&T, where managers are challenged to make science driven and ecosystem bases decisions. Supporting studies are looking at the connectivity of reefs and LBS Pollution management.