Coral reefs, known as the rainforests of the sea, provide humans with food, entertaining, medicines, and coastal protection. The provisions we get from this highly diverse ecosystem are threatened by human and natural factors. Amongst the natural factors, high sea temperatures caused by stronger than usual El Niño events are believed to be the most dangerous. High sea temperatures can cause coral bleaching, a condition where corals lose their symbiotic algae. When sea temperatures remain high for extended periods of time, like during a strong El Niño, corals can die as they cannot live for long periods without their symbiotic algae.
Three severe El Niño events have been linked to three severe global coral bleaching events. In 1998, a severe El Nino event resulted in a 30% coral mortality due to bleaching in the reefs of the Western Indian Ocean. The story repeated itself in 2010, this time coral mortality due to coral bleaching reached 80% in many parts of the world, including the Caribbean. The 2015-2016 El Niño event raised the alarm for another global coral bleaching event that wipe out up to 50% of the remaining coral reefs in the Great Barrier Reef alone.
Here, I shared my bleaching observations, made during visits to coral reefs in three main ocean basis. The visits span a period of 5 months. My observations provide some contrasting results and suggests that we are still far from understanding the response of coral reefs to climate change.
Western Indian Ocean
On October 2015, a harmful algal bloom (HAB) hit the inner granitic islands of the Seychelles. Harmful algal blooms are known to cause serious negative impacts to coral reef communities. Therefore, I worried a lot when I knew it had reached Cousin Island Special Marine Reserve. This no-take marine protected area is the site for the Reef Rescuers Project, the largest coral reef restoration project attempted thus far. At the time, I was the Technical/Scientific Officer of a team who transplanted more than 24 000 nursery-grown coral colonies in 5 000 m2 of degraded reef. I was particularly worried for our restoration site as we had put a lot of effort in restoring the ecosystem services.
The main concern for reef managers considering reef restoration is whether the transplantation of our nursery-grown corals enhances coral resilience and resistance to bleaching. A very relevant concern, particularly now that we were facing the third global bleaching event due to a severe 2015 El Nino. The corals that we transplanted were fragments from donor colonies that survived the first and second global bleaching which we believe should increase the resilience potential of the reef under future bleaching events. Although we can only test our hypothesis once the El Nino threat disappears, our preliminary observations during the bleaching seasons in Seychelles, March to May, suggest that our nursery-grown transplants recover better from temperature-driven bleaching than naturally-grown corals. But, we still need more data to support this statement. Watch this space for news on this subject!
Interestingly enough, nursery-grown corals also responded to a harmful algal bloom (HAB)-driven bleaching better than naturally-grown corals. The bleaching we have seen thus far was temperature related. Coral bleaching caused by HABs is not temperature related but a consequence of a drastic reduction in O2 and photosynthetic active radiation (PAR) during the HAB event. In October 2015, a large-scale HAB was observed in the inner granitic islands of the Seychelles. Recently dead coral colonies were observed a week after this event. Analysis of underwater survey data revealed that the algal bloom caused extensive bleaching mortality at the Healthy Site. The percentage of recent dead coral at the Healthy Site was on average 9.5 % (± 3.7 SE) and reached up to 16% within a transect. This corresponds to a year-on-year 700% increase in recent dead coral cover; it averaged 1.5% in November 2014. We also found that coral transplants responded better to the stressful conditions caused by the HAB. No dead colonies were observed at the Transplanted site. This finding is remarkable. I don’t rule out bleaching at the Transplanted site but for some unknown reason they appear to recover faster and better than corals at other sites. This differential survival to coral insults provide another explanation to the increase in coral cover seen at the Transplanted site compared to the two control sites. Together, our observations show an extraordinary response of an “engineered” site. This is a very promising result that adds support to the premise that transplanting bleaching resistant colonies enhances the resilience potential of coral reefs in the face of climate change.
Eastern Pacific Ocean
Coral reefs in the Eastern Pacific Ocean are scarce and the very few reefs that can be found in the Pacific coasts of Colombia, Panama, and Costa Rica were severely affected by the 1982-83 El Niño. Natural reef recovery has been very slow in the Eastern Pacific most likely due to the reefs being small and very dispersed, dominated by small branching colonies, and a weakly development of crustose coralline algal pavements. This suggest that coral reefs in the Eastern Pacific are doomed under current climate change scenarios i.e. more severe bleaching events.
Well, in February 2016 I witnessed how the 2015-16 El Niño may have killed a large portion of the coral reef area along the Pacific coast of Panama. During the 2016 Rainforests, Reefs and Lost Worlds Noble Caledonia expedition I was lucky to visit the islands of Coiba and Granito de Oro in Panama. I was very keen to get in the waters around these islands and check the reefs for two reasons. On the one hand, a very good friend of mine, Francisco “Pacho” Martinez, an experience diving instructor and owner of Dream Diving Panama, told me a few days before the expedition that the corals in Coiba were in really bad shape. On the other hand, I have not seen Pacho for almost 10 years and our encounter was going to take place doing what we both love: diving! Pacho was not wrong. Nearly 95% of the corals were recently dead from bleaching. They were still white suggesting mortality has occurred just a few days before I arrived. Despite this sad picture, I was happy to see my friend Pacho in the water again. Let´s hope that the remaining 5% that didn´t die comprises “super” corals that are enough to initiate the recovery process. I am returning to Isla Coiba next year to see Pacho and to check the reefs for signs of recovery. I´ll let you know what I find.
Atlantic (Caribbean) Ocean
Coral reefs in the Caribbean are believed to be the least resilient of all coral reefs. Due to over fishing, severe water pollution and coastal development, reef degradation in Caribbean reefs is believed to be greater than in other parts of the world. In addition, once you hear scientists referring to the third and most severe global bleaching event taking place, you expect to see dead reefs everywhere you go. Thus, I was happily surprised when I got in the waters of Guanaja Island in Honduras. What I found below the surface, was unexpected, given the circumstances: large healthy colonies of Elkhorn, Pillar and Star corals; no bleaching observed; no clear signs of recent mortality. There were very few fishes but this is the norm in the Caribbean.
I asked the locals about coral bleaching and, to my surprise, they told me that they haven´t seen bleaching for a few years. Later, I have been asking colleagues about bleaching in the rest of the Caribbean and their feedback is very encouraging. Specially, for the many projects aimed at restoring Caribbean coral reefs. It seems that there are still places where one can find healthy, structurally complex reefs in this region. I will start planning trips to look for them!
The take home message
Coral reefs are remarkable structures. Corals are incredible creatures. The more we learned about them, the more we realised we know so little about them. No all corals will respond in the same way to global or even local events. More importantly, there are places where corals are dying and other places where they are thriving. We need to understand that this is nature and, with or without us, coral reefs will change in configuration, they will transform into new ecosystems. We need to adapt to it and assist coral reefs, as much as we can, in the transformation process, so that they can become sustainable ecosystems. It is our responsibility after being one of the accelerating factors of their change. We need to do it so we can continue benefiting from their ecosystem services.