Coral Bleaching: Threats for Tobago and the Lesser Antilles

Ocean temperatures are reaching dangerous levels for coral reefs in the Caribbean, and international agencies have issued warnings. Dr Anjani Ganase, coral reef specialist, explains what these warnings mean for Tobago.

What is coral bleaching?

Corals form the foundation of our reef ecosystems by providing homes for an array of marine life. Corals are capable of building massive underwater structures because they form a special relationship with the microalgae that live inside the tissues of the coral. This is why corals are found in shallow well-lit tropical waters; the algae can photosynthesise using the sunlight and produce enough energy to supply both themselves and their coral host. In return, the coral provides the algae with a safe haven, storing the essential nutrients and removing the algae’s waste. The presence of the algae in the coral’s tissue makes coral reefs colourful and not transparent like their jellyfish cousins. 

The breakdown of this relationship is referred to as coral bleaching. It typically occurs when the surrounding conditions of the coral are no longer conducive to the coral-algae relationship. When water temperatures are too warm or too cold, or when the water column becomes too cloudy, or if there is an infection, the coral or the algae may not be able to function properly and this results in the expulsion of the algae from the coral tissues. The loss of the algae means that the corals turn white, therefore coral bleaching. The loss of the algae means that the corals lose their major source of food; and if unfavourable conditions persist the coral will starve and eventually die. In the last 40 years, coral reefs around the world have suffered from coral bleaching events, where large sections of reef were turning white as summertime sea temperatures became too warm because of global warming. Coral reefs of Tobago have also suffered from multiple bleaching events, and these have significantly degraded the reef health. 

What does coral bleaching looks like?  Coral colonies bleaching in Utila, Honduras during a 2018 coral-bleaching event.  Photo credit: The Ocean Agency/ Coral Reef Bank.

What does coral bleaching look like? 

Caribbean corals typically are shades of brown and green, but once bleaching begins, the corals appear pale and eventually fade to white. At this point, the corals have lost all their algae pigments and look bone white because we can see their skeletons below their transparent tissue. On some occasions, corals may even appear to glow because of the fluorescent proteins that also occur in their tissue. As bleached corals cannot survive without their algae - the source of food - they become prone to diseases and in many cases they die. Once corals die, turf algae are the first things to grow over the coral skeletons and the reefs will begin to take on a furry appearance as the turf algae take over. Even if corals do not die immediately, they become vulnerable to infections and diseases. Coral diseases tend to spike after stress events. Look out for corals with black bands or patches of white around the coral and partial mortality; these are likely to be infected by disease. 

It’s getting hot!

In the past week, the National Oceanic and Atmospheric Administration (NOAA) have posted the coral bleaching outlook for the wider Caribbean region for the next four months; things are heating up. It is the height of summer in the Caribbean and sea surface temperatures have already risen to above average conditions. Sustained exposure to warm waters, above the functional range for corals, may result in thermal stress in corals and other reef organisms that may then result in coral bleaching and death. NOAA uses observed sea temperatures from satellites, along with historical data and modelling to forecast the likelihood of bleaching in the next four months. From August to November, forecasts indicate coral bleaching stress at 60% likelihood for the wider Caribbean region including the Lesser Antilles and Tobago. Only the presence of storms within the region to stir up and cool the water column will be able to reduce the possibility of a coral bleaching event. 

For Trinidad and Tobago, NOAA’s coral bleaching stress gauge is set to “Warning Level.” This means that the waters around Tobago have been observed as a “hot spot” with water temperatures above the average temperature for this time of year (maximum monthly mean temperature ~ 28 C). If water temperatures are maintained above this temperature or rise further, the likely result is thermal stress in corals and coral bleaching. This is what has been predicted for Tobago over the next 9 – 12 weeks. Parts of Tobago, along with the rest of the Lesser Antilles, will be expected to reach “Bleaching Alert Level One” which corresponds to widespread bleaching.

While the satellite technology of NOAA and their modelling can provide broad predictions, there are limitations in pinpointing which reefs will bleach. Satellites have limited spatial resolution. In addition, other environmental variations, coastal conditions and ecology, will determine whether a reef will bleach or not. The only way we will know if bleaching is occurring is by observing our coral reefs directly. 

Coral bleaching outlook for August to November by the National Oceanic and Atmospheric Administration (NOAA). Areas highlighted on this map show a 60 % likelihood of bleaching. The severity of bleaching is indicated by different shades of red. Photo source: NOAA satellite and information service.

Let’s keep an eye on our coral reefs!

Over the next three months, it is important for those who work or play in the waters around Tobago to keep an eye out for changes to our reefs; and when possible, to record these changes and notify local authorities of changes. In the southwest of Tobago, think about Coral Gardens in Bon Accord Lagoon that is essentially a field of star boulder corals that sit in very shallow waters (3 – 5 m). They would be vulnerable to higher water temperatures, especially a midday low tide on a sunny day. If you paddle, kite surf or snorkel by Coral Gardens or even on a glass bottom boat, take note of the changes to the coral below the surface. Mt Irvine reef fringes one of the most popular surf spots in Tobago; Mt Irvine has great wave formation. While paddling out or riding a wave in, look out for pale or white shapes below the water. Dive tour operators can record the changes on the reef daily by simply making notes or using a GoPro for snapshots. It is important to share your observations with government agencies – The Department of Marine Affairs and Fisheries, THA, the Environmental Management Authority, research institutions such as the Institute of Marine Affairs, and even NGOs in your area - ERIC, BRT and ET to let them know what you have observed. Only then will a more systematic assessment be made about the severity and extent of the coral bleaching.

Reef Recovery

Assisting in the recovery of coral reefs after a bleaching event is the most important process that individuals, communities and governments need to be involved with. In order for corals to recruit and regrow on the same reefs, other forms of stress – typically, fishing, pollution and sedimentation – need to be removed. After bleaching events, corals need to be placed in an intensive care programme. Temporary closure of the impacted reefs to tourism and fishing, plus more stringent restrictions to coastal construction, dumping of sediment, dredging and pollution are necessary measures. Curbing activities that are likely to adversely affect an already vulnerable reef is fundamental. Considering how much Tobago relies on coral reefs for our income, it is paramount that we do all that we can to allow our reefs to recover.

Links to NOAA’s coral reef watch for Trinidad and Tobago:
https://coralreefwatch.noaa.gov/vs/gauges/trinidad_tobago.php

Coral Reef Watch:
https://coralreefwatch.noaa.gov/vs/map.php?lat=20&lng=-76&zoom_level=4&fbclid=IwAR0lKj0rcKcrJexh88xm_BicTnS3OMdTTEtNgpvkYFJioZZoAAKtKp87Rw4