Coral Bleaching 101 Presented by Mark Eakin Coordinator, NOAA Coral Reef Watch

What is a coral? -A “coral” is actually a “coral colony” -Rocky limestone base -Surface is covered by thousands of tiny coral animals, called “polyps” -Polyps are filled with microscopic algae

Anatomy of a coral polyp -Each polyp looks like a tiny sea anemone -Ring of stinging tentacles around a central mouth -Rests in a “cup” on the surface of the colony

What do corals eat? -Like a sea anemone, the polyp can capture small prey (mostly zooplankton) with its tentacles -Also get nutrition from a symbiotic relationship with zooxanthellae….

What are zooxanthellae? AlgaeFish Shrimp

What are zooxanthellae? -Algae that live in the coral polyp’s surface layer -Algae get nutrients and a safe place to grow -Corals get oxygen and help with waste removal -Corals also get most of their food from the algae Photo: Michael ten Lohuis

What is coral bleaching? Coral bleaching is caused by stress Poll question: what might be stressful for a coral? We’ll get back to the answer later…. Healthy coral Bleached coral

What turns the coral white? - As a stress response, corals expel the symbiotic zooxanthellae from their tissues - The coral tissue is clear, so you see the white limestone skeleton underneath Healthy coral with algae Bleached coral with no algae

What can stress a coral? -High light or UV levels -Cold temperatures -Low salinity and high turbidity from coastal runoff events or heavy rain -Exposure to air during very low tides -Major: high water temperatures Photos: AIMS and GBRMPA

Thermal stress -Corals live close to their thermal maximum limit -If water gets 1 or 2°C higher than the summer average, corals get stressed and bleach -NOAA satellites measure global ocean temperature and thermal stress

How warm is warm? How hot do you think the ocean has to get before corals start to bleach?

Can corals recover? -Yes, if the stress doesn’t last too long -Some corals can eat more zooplankton to help survive the lack of zooxanthellae -Some species are more resistant to bleaching, and more able to recover Photos: AIMS and GBRMPA

Can corals recover? -Corals may eventually regain color by repopulating their zooxanthellae -Algae may come from the water column -Or they may come from reproduction of the few cells that remain in the coral Jeff Miller, National Park Service

Can corals recover? -Corals can begin to recover after a few weeks Jeff Miller, National Park Service

Does bleaching kill corals? -Yes, if the stress is severe -Some of the polyps in a colony might die -If the bleaching is really severe, whole colonies might die -Bleaching in Puerto Rico killed an 800-year-old star coral colony in 2005

What else can stress do to corals? Question: what is something that happens to people when they are highly stressed? Photo: Caroline Rogers, USGS Gain weightGet sickTurn orange

Question: what is something that happens to people when they are highly stressed? Photo: Caroline Rogers, USGS What else can stress do to corals? Answer: more likely to get sick

Bleaching and coral disease -Coral diseases are found around the world -High temperatures and bleaching can leave corals more vulnerable to disease -Can quickly kill part or all of the coral colony Marilyn E. Brandt, University of Miami

Bleaching and bioerosion -We have seen that bleaching can kill part or all of a coral colony -Areas of dead coral are more vulnerable to bioerosion (when animals wear away the coral reef’s limestone structure)

Hurricanes & coral bleaching The same warm water that causes corals to bleach can also lead to strong hurricanes. 2005: Most named storms Most hurricanes Most damage in US

Hurricanes : a mixed blessing -Hurricanes can cause direct physical damage to coral reefs -However, the storms also cools the water -Hurricane Katrina led to less bleaching in the Florida Keys, Sept Aug 2005

Hurricanes: a mixed blessing Each passing hurricane in 2005 cooled the water in the Florida Keys. Dennis 9 July Katrina 26 Aug Rita 21 Sept Wilma 24 Oct Sombrero Reef, FL SST (degC) DHW (degC week)

Can we protect corals from bleaching? –Shade reefs –Cool reefs –Improve water quality –Reduce other stress –Reduce overfishing

What Can YOU Do? 1. Learn more about coral reefs. 2. Spread the word.

Bleaching 101: summary - Corals are mineral, animal, AND vegetable - Hot water bleaches corals - Corals may die after bleaching - Diseases follow many warming/bleaching stress events