Major Coral Bleaching Event

October 12, 2016

On the heels of the mass mortality event in July 2016, the corals of NOAA's Flower Garden Banks National Marine Sanctuary are now faced with a separate major bleaching event. Sanctuary researchers report that almost 50% of the coral colonies in a study site at East Flower Garden Bank were bleached or paling (in the process of bleaching) as of October 5, 2016. Sea surface temperatures have been over 30°C (86°F) for 85 days of the past four months, putting the sanctuary in the midst of one of the worst coral bleaching events on record. While the past week has seen temperatures drop slightly to 29°C (84°F), if these warm water conditions continue, the corals may starve and die.

View of coral reef with half of the corals stark white or turning white and the other half normal brownish green colors
Bleaching and paling coral colonies at East Flower Garden Bank on October 5, 2016. Image: FGBNMS/Hickerson

Coral bleaching happens when corals expel their symbiotic algae (zooxanthellae) as a result of some kind of stress event—in this instance, persistent elevated water temperatures (over 30°C or 86°F).  The algae living in the tissue of the coral polyp provide food (through photosynthesis) and give the coral their color. So, when the algae are not present, the tissue of the polyps is mostly clear and the white skeleton of the coral shows through. This creates the bleached, or stark white, appearance. It also leaves the corals without a majority of their food supply.

Close up view of fleshy white polyps of a Spiny Flower Coral.
Close up view of bleached Spiny Flower Coral (Mussa angulosa) polyps from 2005 bleaching event. Image: FGBNMS/Hickerson

The sanctuary’s coral reefs have experienced coral bleaching in the past. Until now, the worst year was 2005 when about 45% of the corals bleached or paled. Other bleaching years of note were 1995 and 2010, but none of these events caused significant coral mortality. Coral scientists agree that increased levels and frequency of coral bleaching are correlated to elevated water temperatures driven by climate change.

A diver floating at the end of a line suspended above bleaching corals on the reef. The diver is holding a reel attached to the line.
Diver reeling in a line near a bleached colony of Boulder Star Coral (Orbicella franksi) at East Flower Garden Bank on October 5, 2016. Image: FGBNMS/Hickerson

Last week, the FGBNMS research team, along with partners from Texas A&M Galveston and Moody Gardens, collected photographs from permanent photo stations within a 100m x 100m study site at East Flower Garden Bank. These stations are photographed annually as part of the long-term monitoring project – a partnership with the Bureau of Ocean Energy Management and the National Marine Sanctuary Foundation. This program has been in place for over 25 years. The value of these repetitive photographs is the ability to track individual corals over time, especially during an event such as this.

Side by side images of the same section of coral reef. The left image shows corals in their normal brownish green colors in July 2016. The right image shows most of the corals are white or turning white in October 2016.
Views of permanent Photo Station 103 at East Flower Garden Bank from July and October 2016. The white and lighter color coral colonies in the October image are bleached or paling. Image: FGBNMS

Preliminary analysis of last week's images from the shallow stations on the reef (60-80 feet deep) show that approximately 46% of the coral colonies were affected (about 24% paling and 22% bleached). At the deep stations (100-130 feet deep), 15% of the coral colonies were affected (11% paling and 4% bleached).

A close view of a brain coral colony that has turned completely white.
A bleached Boulder Brain Coral (Colpophyllia natans) at East Flower Garden Bank on October 5, 2016. Image: FGBNMS/Hickerson

The research team will return to the sanctuary soon to collect data at West Flower Garden Bank, and will continue monitoring the bleached corals over the coming months. Hopefully the stressful temperature conditions will ease and the corals will recruit or replace their symbiotic algae, allowing them to recover. Looking at recent sea surface temperatures, scientists are hopeful that things are starting to cool off. Only time will tell.

A section of reef with a lot of bleached brain coral interspersed with some paling and bleached star corals and some healthy corals of the same varieties.
Bleaching and paling coral colonies at East Flower Garden Bank on October 5, 2016. Image: FGBNMS/Hickerson
A close view of a Mountainous Star Coral colony with the polyps mostly white. Alongside is a completely white colony of another star coral species.
A bleached Great Star Coral (Montastraea cavernosa) at East Flower Garden Bank on October 5, 2016. Image: FGBNMS/Hickerson

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