Ever been so scared that you can feel the color draining from your face? People turn “white as a sheet” because the blood is rushing away from their brains and back to their hearts. It’s almost like your brain expelling the blood because it knows another part of the body needs the blood more — or perhaps it momentarily has less use for it.
A similar phenomenon happens when coral reefs are stressed out by warmer, more acidic waters: their loss of color (known as bleaching) is caused by the expulsion of algae that live inside their bodies. I know that is a little hard to visualize, but think of these algae, called zooxanthellae (zo-o-zan-THE-lay), like a coral’s blood vessels. The zooxanthellae have a symbiotic relationship with the coral in which they live: in exchange for protection, these little organisms filter the water and feed the corals. But what happens when these algae are discharged from the corals? Can they find their way back? How does the coral survive in the meantime?
As mentioned before, the United Nations’ Intergovernmental Panel on Climate Change (IPCC) published its most recent findings on the state of Earth’s climate last month. Last week we learned about how ocean acidification affects organisms at the bottom & top of the marine food chain. After reading an article in The Guardian about coral bleaching, I reread the IPCC’s Small Islands chapter & learned just how great a threat this phenomenon is to coastal settlements and remote island communities. The increase in its frequency and scope in recent years means that bleaching now poses a huge risk to our global society. So without further ado…
What is coral bleaching, and why does it happen?
Corals are colonies comprised of thousands of individual organisms called polyps, which are housed inside calcium carbonate exoskeletons. These polyps form a symbiotic relationship with the algae species zooxanthellae, which reside inside the coral polyp’s cells. How do they get in there? It depends on how the corals reproduce. Some scientists believe the algae is contained within the reproductive cells or donor tissue, and others think polyps suck up zooxanthellae from the surrounding water as they begin to establish a colony. Regardless, the coral provides protection for the zooxanthellae; in exchange, the algae photosynthesizes to convert the energy from sunlight into nutrients the coral can use.
Coral bleaching is the loss of color after corals expel their zooxanthellae into the surrounding water. This loss of pigmentation is caused by warmer temperatures, light or water pollution, or any other changes to the marine status quo. The corals become too stressed to eat or breath; while they remain under this stress they do not permit the zooxanthellae to reenter their cells. Without the algae, the coral loses its main food source (and color source) and becomes more susceptible to disease. While corals do not die immediately during a bleaching event, their chance of survival decreases each day the zooxanthellae cannot return to the polyps.
Ocean temperature changes combined with acidification will lead (and are leading) to widespread bleaching events. Remember: acidification causes shells made of calcium carbonate to disintegrate. If coral polyps have expelled their food source & their skeletal homes are weaker, they will be less protected and more prone to disease, predation, and death. The longer waters remain hotter or more acidic than normal, the longer a coral will be stressed. Higher stress levels lead to more extreme, frequent, and pronounced bleaching events. While some coral species are naturally resilient, others will die.
Is this bleaching fatal to other marine life?
Unfortunately yes. Reef ecosystems are some of the most diverse on the planet yet only make up 2% of the ocean floor. Over a quarter of all marine life live in, on, or around coral reefs. Yet if corals are becoming weaker and dying more quickly and dramatically, the biota (fancy science word for “life forms”) will lose their habitat. They will be forced to migrate, and only if they find a suitable replacement place to live will their species be saved.
It is important to realize that even if reef fish and plankton were immune to climate change-related problems, these organisms would still struggle to survive. This is because reef environments are highly dependent on color, so prolonged bleaching could lead to the collapse of the entire underwater ecosystem. A 2009 study that investigated the effects of coral bleaching on coral reef fishes’ susceptibility to predation found that predation rates on coral-dwelling fish were 17% higher on bleached or dead coral colonies.* According to the abstract, the predatorily fish were almost twice as likely to strike at prey still living in the bright white colonies of bleached corals. Because their habitats had been altered dramatically and quickly, the colorful reef fish were more visible and exposed. Interestingly, predation rates were highest on “algal-covered corals,” which are colonies that have further degraded after bleaching & are now covered with a blanket of different species of neutral or dark colored algae. These scientists conclude that
even if predation does not cause increased in situ[immediate] mortality, it is likely that increased exposure to predators will provide significant motivation for coral-dwelling fishes to vacate bleached coral hosts.
*Note that this study was completed before a dramatic increase the frequency of bleaching events. Imagine how much more vulnerable fish and polyps are now…!
What has coral bleaching looked like in recent years?
Bleaching events have gotten worse and more frequent. Just today, The Guardian reported that the Great Barrier Reef is currently undergoing its 4th major bleaching event in the past six years (there have been at least six such events since 1998). Australian scientists began to worry when ocean temperatures rose to record high levels — 1 to 2°C above average — in the month of December. The level of heat stress over the GBR tends to peak at around this time each year, meaning that the effects of this extra warming were not seen until now.
Furthermore, the IPCC report concludes that small islands are at the most risk due to coral bleaching. Why? Because marine ecosystems will continue to deteriorate and put islanders at higher risk. Coral reefs are a natural barrier, a line of defense between the raw energy in the seas and the civilizations on land. During severe bleaching events, not only do reefs lose a significant amount of coral cover, but they also experience a decrease in growth potential (reef erosion > reef accretion). Scientists observed an increase in sediment production within small islands and atolls in the Pacific & Indian Oceans during the years when bleaching events occurred. This is because the corals could not stop erosion, and the waves from strong ocean storms damaged both the corals and the islands’ shorelines (think of our discussion about sea level rise two weeks ago). The corals’ resilience and adaptability will be crippled by bleaching. Coral colonies are becoming weaker & weaker barriers, like cracked levees or rotted picket fences.
What will coral bleaching look like in the future?
Simply put, coral bleaching events will become more frequent. Above 1.5°C warming, warm water coral reefs will reach their limits of natural adaptability and will begin to die out. When the earth’s temperature increases beyond 1.5°C, small islands will lose 70-90% of their corals; this number increases to 99% if we hit 2°C warming. The warmer the ocean gets, the higher the threat level is to reef ecosystems — and the greater number of these ecosystems will be threatened. The graphic below shows that as the global surface temperatures rise, the first marine environment to show effects of this is corals and coral reefs:
The report states that less than 8% of the ocean is considered a protected area, but even the spots that are protected have “insufficient stewardship” which results in the area being less resilience. Global temperatures have increased by 1.09°C in the past 100-150 years. Even if emissions are substantially curbed in the next decade or two, warming will reach or exceed 1.5°C by 2040. Limiting this overall warming to 1.5°C will mitigate some risks but will not eliminate them. Any continued warming in the next 20-40 years, coupled with increased frequency, severity, and duration of extreme events like coral bleaching, will cause biodiversity loss in marine ecosystems.
So coral bleaching is bad for little remote islands. Why should we care?
Building resilient communities and environments is already proving difficult, but if the climate warms above 2°C it will be impossible in some regions of the world. Between 3.3-3.6 billion people live in places that are vulnerable to climate change. Climate change does not affect people equally because of governance, colonialism, and limited access to resources — all three of these factors play a role in at-risk communities’ responses to global warming. Current mitigation strategies may make the underdeveloped nations even more vulnerable to climate change. The report uses Tuvalu, an island nation of fewer than 12,000 people, as a case study. Tuvalu and other islands in the South Pacific have tried to preserve their land by building sea walls and other structures, which unfortunately have a negative effect on coral reefs and other natural barriers to erosion and flooding. These walls take up space in places where corals can survive, and with populations already threatened by acidification these organisms need all the space (and help) they can get!
Scientists believe we have reached a ‘tipping point,’ which is a threshold beyond which an abrupt or rapid change in a system occurs. Tipping points that have already been reached in ocean systems include the melting of sea ice in the Arctic (a topic I touched on in a previous newsletter), thermal bleaching of tropical coral reefs, and the loss of kelp forests. Once a coral reef has been affected by bleaching, it can take decades for corals to grow back even if temperatures remain below the bleaching threshold. Crossing a tipping point can cause entire populations to collapse, resulting in local & regional extinction events. Thus human-induced climate change will continue to force ecosystems into abrupt and often irreversible change unless we take strong mitigation and adaptation.
So what can we do?
Human beings need to reinforce coral reefs and/or develop mitigation strategies so that the ecosystems will not collapse. The IPCC urges governments and nations to transition from “incremental to transformational adaptation.” If drastic changes are not made soon, everything will get worse at such a rate that our current ideas will become ineffective by 2040. The report stresses that adaptation does not prevent all loss and damages: some strategies that work in the short term could prove harmful in the long term, strengthening the negative effects of climate change. Short-term planning, especially regarding coral reefs, does more harm than good. Any adaptation efforts like sea walls must be part of a larger plan; otherwise, they will both be ineffective and harm the new environments that are developing around them.
That being said, IPCC scientists believe that coral bleaching can be mitigated by “pre-exposure to elevated temperatures.” There is great variability between different coral species across all regions of the world, and some corals are better equipped either genetically or due to their environment to handle heat waves. In fact, there have been reports of resurgences in the GBR’s coral colonies, although not enough to offset the harm done by previous bleaching events. Some areas of the ocean are less travelled by humans, so these reefs are not as stressed out as others and therefore are better able to adapt to warmer temperatures. Transplanting heat-tolerant coral colonies to vulnerable areas can increase reef resistance to bleaching, but it also could potentially lower species diversity and fundamentally change the nature of an established ecosystem.
Currently, scientists are trying to genetically modify corals so that they can withstand higher temperatures. They are also trying reef shading techniques: this process essentially involves putting a tarp-like thing or increasing turbulence over parts of a small reef in order to reduce solar radiation & subsequently stress on the wee polyps during a marine heat wave. However, the report emphasizes that many adaptation strategies which prove successful in the lab are impractical because they cannot be scaled up to save corals in the wild. Still, it is encouraging that progress has been made to protect and preserve the ‘rainforests of the sea.’
Sources not linked above:
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-IPCC, 2022: Annex II: Glossary [Möller, V, J.B.R. Matthews, R. van Diemen, C. Méndez, S. Semenov, J.S. Fuglestvedt, A. Reisinger (eds.)]. In: Climate Change 2022: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press. In Press.
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