Updated: 6 days ago
By Ally Roberts
Coral Reefs are a treasured part of our ocean systems. They not only drive our economy by supporting tourism, they are a pivotal part of environmental stability. They act as a barrier between the oceans and the shoreline, helping to soften the blow of larger waves and storms. However, we are losing these reefs at an alarming rate. As of 2013, we have lost 19% of the total coral reef cover, and upwards of 80% solely in the Caribbean. (Miller and Richardson 2013)
Coral Reef Biology
Coral Reefs are composed of Anthozoans and Porifera
Anemones, bony and hard corals
belong to the same phylum as jellyfish
lack true tissues and organs
Main group affected by BBD
What these corals rely on is symbiotic, or mutually beneficial relationship, with Zooxanthellae algae. This algae utilizes the carbon dioxide and nitrogenous waste produced by the corals, and in return the algae protects the outer layers of the coral. Everyone has heard of coral bleaching, which is when they expel these Zooxanthellae algae and begin turning white. Although this event is extremely harmful, it can be survived. Many point to global warming, or the rising temperature in our oceans, as they culprit behind coral bleaching. Today, we will look at a driver of coral decline that goes hand in hand with these increasing temperatures
Black Band Disease, or BBD, was the first coral disease to ever be discovered. According to a paper written in 2004, BBD is easily distinguishable by the dark microbial mat it creates, which leaves a noticeable band on the coral as it pulls the living tissue from the organism. (Sutherland et al. 2004) This mat eventually forms a ring and moves across the reef, spreading to new hosts and eventually decimating whole reef systems. A group of bacteria called Vibrio sp. have been shown to turn pathogenic when subjected to higher temperatures. (Miller et al. 2013) Meaning, the warmer the oceans get, the more often this temperature induced conversion will occur.
Drivers of Coral Decline
Our weather cycles regularly switch between 2 different oscillation cycles: El Niño and La Niña. During the El Niño cycle, sea temperatures generally rise combined with lower sea levels. This oscillation cycle is already associated with higher sea temperatures and Marine Heat Waves, combined with the lower sea levels this effect is only magnified as sunlight can more easily reach corals on the ocean floor. According to Miller and Richardson in 2013, BBD shows a positive correlation with increasing temperature in the area of prevalence, progression rate and transmission. Meaning that when ocean temperatures are higher, this already deadly disease progresses faster, transmits easier and is more wide spread than usual.
The Path Reefs Are Heading Down
Global temperatures are continuing to rise, and with this the average temperatures of our oceans. According to the Environmental Protection Agency (EPA) ocean surface temperatures have been steadily climbing since about 1975. with no sign of slowing down.
Therefore, coral bleaching and related factors like BBD are only going to continue to plague coral reefs across the globe. Coral reefs act as a sanctuary for aquatic diversity, and many fish species utilize them for protection and reproduction. Without them, they would have to quickly to adapt and find new reservoirs to inhabit, which is extremely challenging. Not only are they vital for wildlife, but coral reefs are a source of protection and economic stability. Large tropical storms would hit coastlines harder without these reefs acting as buffers, and their position in tourism makes them a huge piece of some major cities economy. With all the human attention they attract, it should be easy to drive conservational efforts for them.
Further research is required in order to devise a defense strategy. There is a bacteria called Ruegeria, which is drawn to Sulfur compounds released by corals under stress from heat. This bacteria is shown to have the ability to protect corals from pathogenic Vibrio. (Portillo et al. 2020) In other words, corals who are experience bleaching or other heat related issues may release these Sulfur compounds which attracts the very bacteria that may be able to protect them. Further understanding their relationship could be the key to preventing any more reef loss.
“Given current predictions of increased human activity in the Caribbean, the growing threat of climate change on coral mortality and reef framework building, and the potential synergy between these threats, the situation for Caribbean coral reefs does not look likely to improve in either the short or long term.” (Gardner et al. 2003)
Changing our own behavior such as: preventing over fishing, stopping habitat destruction and halting agricultural run off; may help to strengthen the reefs further. Coral reefs protect our coastlines every day, we should be a part of the team that helps to protect them back.
Ampou, Eghbert Elvan, et al. “Coral Mortality Induced by the 2015–2016 El-Niño in Indonesia: the Effect of Rapid Sea Level Fall.” Biogeosciences, vol. 14, no. 4, 2017, pp. 817–826., doi:10.5194/bg-14-817-2017.
Gardner, Toby A., et al. “Long-Term Region-Wide Declines in Caribbean Corals.” Science, vol. 301, no. 5635, 2003, pp. 958–960., doi:10.1126/science.1086050.
Miller, Aaron W., and Laurie L. Richardson. “Emerging Coral Diseases: A Temperature-Driven Process?” Marine Ecology, vol. 36, no. 3, 2014, pp. 278–291., doi:10.1111/maec.12142.
Rubio-Portillo, Esther, et al. “Shifts in Marine Invertebrate Bacterial Assemblages Associated with Tissue Necrosis during a Heatwave.” Coral Reefs, vol. 40, 2021, pp. 395–404., doi:10.1101/2021.01.25.428091.
Sutherland, KP, et al. “Disease and Immunity in Caribbean and Indo-Pacific Zooxanthellate Corals.” Marine Ecology Progress Series, vol. 266, 2004, pp. 273–302., doi:10.3354/meps266273.