Have you ever stepped outdoors and immediately felt a sweltering heat that made you feel to run back inside and escape to an air-conditioned room or a cold shower? Living in Trinidad and Tobago, I’d bet that’s true. Except, imagine if there was no way to get relief from that heat; this is what the corals around Tobago are currently experiencing as sea surface temperatures rise above normal level.
Why are sea surface temperatures increasing and how does warmer water affect corals?
As global warming continues, trapped heat within the Earth’s atmosphere is absorbed by the ocean. This results in sea surface temperatures becoming hotter, negatively impacting coral reefs. Corals are fascinating, immobile invertebrate animals that require specific environmental conditions for their growth and survival. They have a very close, mutualistic relationship with microscopic algae, also known as zooxanthellae, that live in the corals’ tissues. These zooxanthellae are very important to corals. Like all plants, the zooxanthellae photosynthesise and the nutrients from this process provides food for the corals which allows them to grow and reproduce, building coral reefs. Zooxanthellae also give corals their brilliant colours!
Like most relationships, however, this one does have a few boundaries. This symbiotic relationship flourishes at sea temperatures around 25°C to 28°C. Persistently high temperatures, that are above 29°C, create stressful conditions for both corals and zooxanthellae. As the water becomes warmer, there is often a loss of zooxanthellae from coral tissue or the loss of pigment of the zooxanthellae itself. This results in a “bleached appearance” in corals as they lose their colours and appear white because their calcium carbonate skeleton becomes exposed. This is known as “coral bleaching”. As corals lose their main food source, their energy and functions are significantly decreased. If the warmer periods do not last long, corals may regain zooxanthellae once the water temperature returns to what is considered ideal. The corals can gradually rebuild their strength. However, when periods of warmer water persist for weeks, the corals are often unable to regain zooxanthellae, the colonies do not get the sustenance they need, and they eventually die.
The dive team at the IMA, along with volunteers, have been monitoring for coral spawning in Tobago since July 2022 as part of the Marine Resilience Initiative (MARIN) Tobago pilot project. Coral spawning refers to the process where corals release their eggs and sperm (gametes) into the water for external fertilization and formation of a larvae. These larvae then settle on the reef and continue growing to form a new coral colony. It has been an eventful few months; we were able to observe spawning of a number of coral species. It truly is a magnificent experience to witness the bundles of eggs and sperm, which resemble snow flurries, being released underwater!
However, the anticipation of this magical experience for the final expedition for the year by the IMA dive team was marred by advisories of bleaching from National Oceanographic and Atmospheric Administration’s (NOAA) Coral Watch in October 2022. With a Bleaching Alert Level 2 issued for Tobago, the IMA Coral Research and Dive Team feared the worst. Here we were, ready to conduct our final monitoring trip and witness the magnificent spawning for potential new baby corals, but instead were dreading the extent of bleaching we might find. Admittedly, as our research is focused on improving the resilience of coral reefs, it is quite challenging as there are multiple factors that threaten the health and survival of these same corals. Interestingly, there are similarities between sick corals and sick humans. When we are unwell, the best recommendation is rest which allows our immune systems to defend against infections and diseases. We often do not have energy for other activities. This is the same for corals. When corals are significantly stressed, their energy budget is used primarily in their immune response, thus decreasing the amount of energy allocated for growth and reproduction. Understanding this, we anticipated that less corals may be spawning on the reefs.
Fortunately, we observed only a few colonies bleaching, and many species still looked healthy. However, the concern for the corals is still high as the Bleaching Alert 2 warning has been extended potentially until December 2022. IMA is asking persons that work in the marine environment to keep an eye out for bleaching. We have received reports of paling and bleaching of brain and branching corals in southwest Tobago. Persons can report sightings of bleaching corals to the Institute of Marine Affairs via the SeaiTT app.
It is important to conduct a coral bleaching assessment later in the year as it is critical to record how the corals are coping. It is also crucial to note that corals that survive bleaching often have weaker immune systems and are more prone to subsequent stresses, such as disease. It is common to observe coral disease outbreaks after bleaching events, another factor that will be monitored in the bleaching assessment. Furthermore, localised threats including land-based sources of pollution, such as sediment and fertiliser runoff increase the stress and further compromise the health of corals.
We were able to observe some corals spawning in October 2022 despite the bleaching alert. This gives us hope, that in this race to save coral reefs, there are still resilient coral colonies that are persevering despite the threats they face and therefore deserve protection and conservation. Corals build reefs that serve as habitats for sea life and give us food. Corals protect our coastal communities from strong, erosive waves. Coral reefs are known to provide aesthetic and breath-taking seascapes from which society benefits not only economically, but mentally and physically as we rely on the sea for healing and restoration. Preservation of corals is of utmost importance as it contributes to our sustainable blue economy.
Hannah Lochan is a Marine Technician working on the Marine Resilience Initiative (MARIN) Tobago pilot project for the Institute of Marine Affairs (IMA). Hannah reports on the team’s observations of coral bleaching, while monitoring for coral spawning in October.