Coral Bleaching in our Backyard

Shivonne Peters, marine scientist, reports on coral bleaching in Tobago’s iconic Buccoo Reef.

Trinidad and Tobago, like other countries in the Caribbean region, is under a coral bleaching watch, the fifth occasion for 2022. As ocean temperatures continue to warm, the Oceanic and Atmospheric Administration (NOAA) in its October report indicated that the country is currently under a Bleaching Alert Level 2, meaning that widespread bleaching leading to coral mortality is expected. These conditions are expected to extend throughout December with reports indicating coral bleaching is currently occurring in reefs along Tobago’s north-east and south-west coastlines. Bleaching of the Fire Corals (Millepora) and Staghorn Corals (Acropora cervicornis) was observed in the Buccoo and Charlotteville reefs earlier this month. Sustained warm ocean temperatures will mean continuing decline and disaster for Tobago’s coral reefs.

Coral bleaching occurs when the coral - marine invertebrates comprising tiny animals called polyps which live together in the hundreds to form colonies - responds to various stressors. One stressor is higher water temperatures, typically a 1.5 to 2^º Celsius increase; another is  poor water quality due to pollution. Excessive sunlight, exposure due to lower ocean tides and sometimes even colder water temperatures can also affect corals.. Ocean warming due to climate change is the leading cause of coral bleaching. As the coral responds to these stressors, they expel the algae (zooxanthellae) living in their tissues. The algae perform a critical function by producing food for the animal through photosynthesis. Once expelled, the coral turns white or pale in colour and the organism becomes more susceptible to disease. Coral bleaching does not necessarily lead to death as corals can survive bleaching events, provided that other environmental disturbances are limited.

 

Suspected coral bleaching in Staghorn Corals (Acropora cervicornis) in South-West Tobago.

Suspected coral bleaching in Fire Corals (Millepora) in South-West Tobago.

 

The Caribbean is not alone in experiencing coral bleaching events. In Australia’s Great Barrier Reef, the 2022 mass bleaching event was the fourth such event in the last seven years. This year, scientists estimated that 91% of the Great Barrier Reef was affected by coral bleaching. In this instance, however, coral bleaching occurred despite cooler-than-normal ocean temperatures across the Pacific Ocean caused by the La Niña weather phenomenon. This year’s mass bleaching event was not as severe as the 2016 and 2017 events which resulted in an approximate 30% coral mortality rate. Despite these extensive coral bleaching events, including those reported thus far for 2022, scientists have recorded the highest coral cover observed in the last 36 years in northern and central regions of the Great Barrier Reef.

In Tobago, coral reefs faced two mass bleaching events in 2005 and 2010. The 2005 mass bleaching event resulted in up to 85% bleaching and up to 75% mortality of important reef-building species such as the Boulder Brain (Colpophyllia natans) and Grooved Brain (Pseudodiploria spp.) (Alemu, 2014). Similarly, following the 2010 mass bleaching event, scientific studies concluded that 60% of corals in reefs at Speyside were bleached with 50% coral loss. Additionally, occurrences of white band disease (caused by pathogens including bacteria entering coral tissue rendered vulnerable by bleaching events) were observed in Tobago following the 2011 mass bleaching and ultimately reducing the chances of coral recovery. Pollution and overfishing, unmanaged in our local marine environment, also limit the natural recovery processes.

One might argue that coral bleaching has no direct impact on us here in Trinidad and Tobago, however, that is in fact far from reality. Tobago relies primarily on the tourism industry as the main driver of its economy. A 2005 study valued Tobago’s coral reef-related tourism industry, including glass bottom boat tours, snorkelling and diving, at US$101- $130 million per year. The Buccoo Reef remains one of the island’s main tourist attractions with reef tours occurring daily and allowing snorkelling in Coral Gardens. The bleaching and mortality of corals in this area and at Speyside can negatively impact this industry. Healthy coral reefs and diverse fish populations are attractions for tourists. If bleaching events - particularly back-to-back and prolonged - continue indefinitely, our marine attractions will be considerably diminished.

Moreover, the value of our coral reefs does not rest solely on their contribution to the tourism industry. Marine habitats such as coral reefs, mangroves and seagrasses dissipate wave energy and protect the shoreline against erosion. These habitats have protected our infrastructure including homes and businesses from the direct impacts of extreme weather events such as storms and hurricanes. Coral reefs in Tobago also sustain our commercial fisheries by acting as nurseries for juvenile species of fish and crustaceans. Not only do these habitats sequester heavy metals from the substrate, they also provide cultural benefits and a host of additional ecosystem services. The slow decline of these functions will lead to social, economic and environmental consequences for us in Trinidad and Tobago.

While we cannot directly stop coral bleaching caused by warming ocean temperatures, we can reduce the stressors which increase the likelihood of coral mortality following bleaching events. Controlling land-based sources of pollution is one such measure. Excessive nutrients stimulate algal growth and heavy sediment loads reduce light penetration and ultimately the coral’s ability to produce its food through photosynthesis. Reducing invasive species, such as the lionfish, can also aid in coral recovery by maintaining native reef fish populations and contributing to healthier reef systems. The Institute of Marine Affairs, in addition to conducting routine coral reef monitoring and research in Tobago, has developed the SeaiTT app where anyone can report any environmental incidents including coral bleaching. Tour operators who frequent the reefs at Buccoo and Speyside should be on the lookout for coral bleaching. As scientists gain a greater understanding of the extent of these bleaching events in Tobago, the better the likelihood of protecting and preserving our precious coral reefs.

(Shivonne M. Peters is a regional consultant specializing in Marine Protected Area Management and Sustainable Development. She is a PhD candidate in Marine Sciences at the University of Trinidad and Tobago. Her thesis focuses on the development of a Marine Spatial Plan for the Buccoo Reef Marine Park. For further information email shivonne17@hotmail.com.)

References

Alemu I, J. B., & Clement, Y. (2014). Mass coral bleaching in 2010 in the southern Caribbean. PLoS one, 9(1), e83829.

Burke, L., Greenhalgh, S., Prager, D., & Cooper, E. (2008). Coastal capital: economic valuation of coral reefs in Tobago and St. Lucia. Coastal capital: economic valuation of coral reefs in Tobago and St. Lucia.