Coral reefs are some of the most important regions of the oceans for marine biodiversity. Globally, coral reefs face a variety of threats. Pollution and overfishing present some of the most obvious problems that threaten diversity and can lead to destruction of reef colonies.
Protection measures in recent years have helped stem some of these threats in places, but we may also need to prioritize this protection to also mitigate climate change effects. A difficult challenge already being faced by coral reefs is rising temperatures in the world’s oceans which can lead to mass bleaching events.
Typically, in El Niño years such as 2023, ocean temperatures increase significantly. Additionally, the oceans for decades have been absorbing much of the increasing temperatures we are facing. While this has perhaps delayed some of the worst impacts of climate change, this has also meant that ocean temperatures are now experiencing unprecedented levels of heat that threaten many species.
What is coral bleaching?
Coral reef bleaching is a phenomenon that occurs when the symbiotic relationships between corals and their resident algae (zooxanthellae) are disrupted. Warmer ocean temperatures are one of the primary factors contributing to this disruption.
The elevated temperatures cause stress to the corals, leading to the expulsion or reduction of the photosynthetic algae. Since these algae are responsible for producing a significant portion of the coral’s food and contribute to the vibrant colors of the corals, their loss results in a pale or “bleached” appearance. Furthermore, the reduction in algae compromises the corals’ energy production, making them more susceptible to diseases and other environmental stressors.
This process has been widely observed in various reef systems around the world and is of considerable concern to marine biologists, as it may lead to the loss of biodiversity and ecological balance within these vital ecosystems.
Ocean warming in 2015 led to coral reef die-off off the coast of Hawai’i
In 2015 off the coast of Hawai’i Island, in a previous year of unprecedented water temperature increase that was 2.2 °C above normal, water temperatures reached 29.4 °C. This led to a quarter of reefs in the area losing 20% of their reef cover or more. The most affected reef lost more than half of its cover, while 18% of the reefs were unaffected or even expanded.
The good news is that reefs that were less impacted by pollution, where water quality was relatively good, were better able to withstand increasing water temperatures. Herbivorous scraper fish, such as parrotfish, help maintain healthy coral by eating algae. This has helped, at least in local places, for coral reefs to do well and even expand as scraper fish removed the algae that enabled corals to build.
However, it is clear that this may not be enough. In 2016-2017 in the Great Barrier Reef off Australia, wide-scale bleaching events led to mass coral dying. This is explained by both increasing water temperatures as well as pollution weakening coral reefs. In 2022, the sixth recorded widespread coral bleaching event since 1988 occurred.
Overfishing and ocean pollution make coral reef vulnerable to bleaching events
Researchers argue that coral reefs will continue to be under threat and face wide-scale bleaching events similar to what occurred in the Great Barrier Reef in 2016-2017. However, this can be slowed down and some reefs can even thrive if pollution in the ocean is reduced and overfishing prevented.
Wastewater pollution and urban run-off make reefs vulnerable, particularly when that pollution contains a toxic mix of various chemicals from agriculture or medicines. Additionally, overfishing near corals reduces herbivorous fish populations because many of the fish that eat algae live near or within corals.
Researchers calculated that there is a three- to sixfold greater probability of a reef having greater reef build, using data from Hawai’i, if healthy fish populations are maintained and pollution is minimal to non-existent. In fact, if there is any dying, these reefs are also better able to recover. While overall it is likely most coral reefs could be affected by higher water temperatures, that loss can be mitigated by maintaining healthy fish populations and minimizing land-based pollution. This highlights that run-off pollution also needs to be much more strictly monitored.
Additionally, data from satellites, including NASA’s Aqua MODIS satellite, is showing sea surface temperature increasing in recent years across the tropics and more temperate regions. This has led to particularly troubling ocean heatwaves that dramatically raise temperatures several degrees celsius during these events.
Poor circulation of ocean water results in higher sea surface temperatures
Using recent work off Moorea, French Polynesia, scientists have demonstrated how ocean eddies weakening internal waves have led to higher temperatures in oceans found at different levels due to poor circulation of colder deep water. Scientists conclude that variability in eddy fields, which affects thermocline depths or the transition between warmer surface water and cooler lower waters, will lead to increasing water temperatures.
This will have the effect of prolonging the timing and severity of marine heatwaves as colder deeper water cannot easily offset increasing surface water temperatures. These effects could be severe enough to not only affect more vulnerable coral weakened by pollution and overfishing, but healthier coral may also be severely impacted if eddies are severely disrupted.
Ocean warming will lead to more coral reef bleaching events
The oceans are incredibly important for our global climate and, to a large extent, have also helped us absorb some of the increasing temperatures our planet faces. However, as water temperatures increase in the oceans, coral reefs will begin to face severe challenges, particularly if they have been impacted by pollution or overfishing.
We can mitigate this damage by better prioritizing the limitation of run-off pollution and reducing fishing near coral reefs. However, scientists also warn that the rates of water temperature increase are also likely to affect most of the world’s coral reefs in the next few decades.
 For more on how coral reef health has been affected by rising water temperatures and pollution, see: Nogrady B (2023) Controlling pollution and overfishing can help protect coral reefs — but it’s not enough. Nature: d41586-023-02512-w. DOI: 10.1038/d41586-023-02512-w.
 For more on the effects of pollution and depleted fish populations on coral health, see: Gove JM, Williams GJ, Lecky J, et al. (2023) Coral reefs benefit from reduced land–sea impacts under ocean warming. Nature. DOI: 10.1038/s41586-023-06394-w.
 For more on marine heatwaves and how this is affected by changes in marine eddies affecting thermoclines, see: Wyatt ASJ, Leichter JJ, Washburn L, et al. (2023) Hidden heatwaves and severe coral bleaching linked to mesoscale eddies and thermocline dynamics. Nature Communications 14(1): 25. DOI: 10.1038/s41467-022-35550-5.