Coral Bleaching

Reefscape with half the corals dead and covered in algae, and the other half of the reef bleached white.

What happened in Summer '23?

Florida's Coral Reef experienced the worst coral bleaching event ever recorded during the Summer of 2023. Coinciding with an El Niño year, unusually hot waters started in mid-July, a month earlier than the typical peak heating months of August and September. Because water temperatures didn't cool off until October, this forced our corals to spend almost double the time in abnormally hot temperatures compared to any of the previous bleaching years in Florida. Check out this Flickr album for images of the bleaching event.

How hot was the water?

Water temperatures were 5 degrees hotter than usual, with temperatures on the reef reaching up to 93°F. Corals typically bleach when water gets warmer than normal - in Florida this is around 87°F. Water temperatures in the 90s are exceptional. Extended periods of time spent at or above this temperature threshold can result in direct coral mortality. The NOAA Coral Reef Watch program has great information on coral bleaching thresholds.

Closeup of a bleached star coral with a dive watch showing the temperature 93.2 degrees.

How did our corals fare?

An underwater photo of bleached and dying elkhorn coral, with bleached white branches and understand and dead, brown upper branches.

Data are still being analyzed from the 2023 monitoring season, but our researchers observed:

Mass coral bleaching, with many reefs having 100% of the corals bleached
Significant population declines in elkhorn and staghorn corals
Some coral mortality of brain, finger, and lettuce corals
Widespread octocoral (soft corals) mortality from heat shock

Corals in the Dry Tortugas, Marquesas, Florida Keys, and southern Miami-Dade County were most heavily impacted. Luckily, the northern part of Florida's Coral Reef (Palm Beach, Martin, and Broward Counties) were not as impacted during this bleaching event.

What is coral bleaching?

Corals are animals that live in a symbiotic relationship with microscopic algae called zooxanthellae that live within the coral tissues. The zooxanthellae get a safe place to live within the corals, and the corals receive energy their zooxanthellae produce from photosynthesis. Corals get the majority of their nutrition (>85%) from this relationship. They can also feed using their tentacles full of stinging cells to capture zooplankton, but this is a small percentage of their diet (<15%) and may not be enough to sustain the corals.

Closeup of a bleached star coral with streaks of healthy brown color radiating from the polyp mouths.

The average American consumes about 2,000 calories per day. Imagine if suddenly your diet was cut by 85% to only 300 calories per day. You could live on 300 calories per day, but for how long?

Closeup of a very pale smooth flower coral, mostly white with pale yellow-brown dots visible on the coral, with tentacles extended.

When waters get too hot, corals lose their zooxanthellae which provide corals their color – when they are gone coral tissues are transparent and you can see their white skeleton through their tissues. The corals now look white, or "bleached". Corals can typically recover from short-term bleaching if they are not exposed to hot temperatures for too long. But, when corals do not have their zooxanthellae for longer than a couple of weeks, they can starve to death.

How often does coral bleaching occur?

The 2023 El Niño event has contributed to the severity of this year's bleaching, but climate change also plays a role in the frequency and severity of bleaching events. Bleaching has been occurring more frequently over the past few decades, and mild to severe bleaching has occurred every year since 2011. The infographic below shows the years when bleaching was documented in Florida and when El Niño concurrently occurred.

Monitoring Programs

FWC has two programs that monitored the 2023 bleaching event. The Coral Reef Evaluation and Monitoring Project (CREMP) monitors 73 sites annually throughout the Dry Tortugas, Florida Keys, and Southeast Florida. The Disturbance Response Monitoring program conducts hundreds of surveys throughout the entirety of Florida's Coral Reef during peak bleaching months (typically August-October). Data will be made available from both programs later in 2024.

A diver holding a clipboard hovering over a bleached white star coral on the side of a steeply sloped reef.

What can we do to help?

The root cause of this bleaching event is global climate change driven by excessive greenhouse gas emissions.
- Reduce your carbon emissions by driving less, being more energy efficient, eating more plant-based meals, buying local, and purchasing carbon offsets when flying.
- Support policies, policy-makers, and programs that reduce carbon emissions.
Land-based sources of pollution - including toxins, sediments, and nutrients - run off from urban areas onto our reefs. Such pollution can impede coral growth, reproduction, and other biological functions, and cause disease and mortality. This chronic stress reduces our reef's ability to be resilient to disturbance events like hot-water thermal stress.
- Follow the principles of Florida-Friendly Landscaping, properly dispose of household products and chemicals, and reduce water consumption.
- Support policies, policy-makers, and programs that address land-based sources of pollution and water conservation - including responsible development and septic-to-sewer conversions.
Support coral conservation and coral restoration, either through financial support or by volunteering.
- Contact your congresspersons and ask them to support federal funding for coral reef
  conservation and restoration.
If you’re out on Florida’s Coral Reef, you can report bleaching, disease, and other impacts through SEAFAN, NOAA Coral Reef Watch, or BleachWatch Florida Keys.