Calm Seas, Hot Weather Are Bad News For Corals

Aug 13, 2015

People who fish and dive love it when the winds are light and the seas are calm.

Coral reefs, not so much.

That's because calm seas means sunlight can penetrate better and heat up the water. When sea temperatures pass a 30.4 degrees Celsius average for a month — that's 86.72 degrees Fahrenheit — corals start to bleach.

A bleached coral at Molasses Reef in August, 2011. When sea temperatures rise, corals expel their symbiotic algae and appear white, or bleached.
Credit Florida Keys National Marine Sanctuary

The corals expel the zooxanthellae, or symbiotic algae that gives them their colors.

"Without that symbiotic relationship, it's unable to feed properly. The coral is a living animal," says Sean Morton, superintendent of the Florida Keys National Marine Sanctuary. "That can lead to stress and high propensity for disease or just coral die-off."

This week, a NOAA monitoring station at Molasses Reef off Key Largo found that sea temperature had passed the bleaching threshold. Last year was the warmest on record for that station.

There is not a lot scientists and sanctuary managers can do to protect coral from bleaching, but Morton says the sanctuary ramps up education efforts when bleaching starts, because corals are even more vulnerable.

"There are a lot of snorkelers and divers coming to the Keys all the time," Morton says. "That kind of human interaction has an even more negative effect in that stressed moment."

This graphic shows 30-day average temperatures collected at a data station on Molasses Reef off Key Largo. The red line shows 30.4 degrees Celsius, or the bleaching threshold. It was exceeded this week.
Credit NOAA

The sanctuary partners with Mote Marine Laboratories in the Keys in a BleachWatch program that collects observations of bleaching. 

Morton says it's important for the public to report where they see bleaching — and where they don't. That way the sanctuary managers and scientists can help measure the extent of the bleaching, and also identify areas where corals may be more resilient.