Disease and heat pose a deadly combination for coral reefs, according to a new University of Georgia study .
Previous research has shown that heat stress can increase disease and kill coral. But the new paper found that corals with preexisting disease conditions were particularly vulnerable.
Coral health is strongly influenced by water quality. Corals growing in poor quality water frequently have more disease that is often more severe than reefs in cleaner waters.
"Water quality protection is a local issue," said James Porter , co-author of the study and the Josiah Meigs Distinguished Professor Emeritus in UGA's Odum School of Ecology . "These results demonstrate that measures to improve local water quality will also improve coral survival."
The researchers also found conservationists can help new coral grow by focusing efforts on the edges of the reefs.
"We should be focusing restoration efforts in places where the corals are most likely to survive," said Camilla Nivison , corresponding author of the study and a doctoral student in the Odum School. "We found some reefs during the heat wave to be an oasis with minimal damage. Nobody's been able to pin down exactly why, but it appears that topography is important."
Hotter, longer heat waves expected to exacerbate effect on coral
The researchers analyzed hundreds of square feet of coral colonies in the Florida Keys in the summer of 2023. While every year experiences high temperatures in late summer, 2023 broke every record. A record-breaking heat wave pushed water temperatures above 87 degrees F for 41 straight days.
Every summer since has experienced similarly hot, prolonged temperatures.
Using high-resolution 3D layered photo imaging and underwater mapping, the researchers found heat stress caused immense coral bleaching.
Bleaching happens when coral is stressed by high temperatures or other factors. Under normal circumstances, beneficial algae live in coral, but when water temperatures get too high, the coral expels the algae, exposing the white skeleton underneath.
Over time, bleaching leaves coral increasingly vulnerable to disease and starvation, as the microbiome of the coral gets disrupted by pollution or heat.
Multiple stressors are not a good thing. When the corals are handling threats from multiple things … that makes it more challenging for them to survive.
Camilla Nivison, Odum School of Ecology
The study found that heat was the primary killer of the coral reefs. But coral with existing disease lesions had a nearly four times higher risk of dying due to the heat than healthy coral.
Healthy coral tissue located next to diseased coral was almost twice as likely to die during extreme heat than healthy coral tissue located further away from diseased areas.
"Multiple stressors are not a good thing," Nivison said. "When the corals are handling threats from multiple things at the same time, unsurprisingly, that makes it more challenging for them to survive."
Previous studies have shown certain types of coral were hit particularly hard by the combination of disease and hot water temperatures.
In the present study, the researchers found elkhorn coral experienced catastrophic losses during the 2023 heatwave.
"This coral used to be the most common coral in the Caribbean," Porter said. "The loss of this coral throughout the Caribbean is comparable to the death of oak trees from Maine to Florida.
"We've been monitoring coral reefs for a very long time, and they are in trouble. Since I began diving in 1972, we've lost more than two-thirds of all living coral worldwide."
Coral located on outer reef edges more likely to survive
The location of the coral had an impact on its survival, the study found.
Corals along the edges of reefs were more likely to survive warmer temperatures. Being on the edge allows water to move faster around the coral, helping protect it against the accumulation of waste while also enhancing feeding opportunities.
"The more water is moving across, the more it's removing things that are diffusing out of the coral, specifically metabolic wastes," Nivison said. "That helps protect the coral tissue, especially when it is stressed by heat."
As conservationists replant coral in restoration efforts to combat these die-offs, the study suggests planting coral near the edges gives them a better chance of regrowing and thriving.
The health of the ocean really affects and impacts the health of coral reefs. As go coastal oceans, so go coral reefs.
James Porter, Odum School
Reducing coastal pollution can also cut down on pathogenic bacteria and nutrient pollution that stress corals and can cause disease.
"The health of the ocean really affects and impacts the health of coral reefs. As go coastal oceans, so go coral reefs," Porter said. "Coral reefs are really important to humankind and the natural world, so the biodiversity of coral reefs is extraordinarily important to both humans and to our living planet.
"You can't change the whole globe, but you can do a lot of work towards mitigating local pollution and improving local water quality."
The study was published in Coral Reefs.
