Diseased corals were nearly four times as likely to die in the Florida Keys' 2023 heat wave

The University of Georgia team behind the Coral Reefs paper ^[6] built high-resolution 3D layered photo models and underwater maps of Keys reefs ^[12], covering hundreds of square feet of coral colonies in summer 2023 ^[10]. Mapping at that scale lets a survey compare tissue with lesions against tissue without them, on the same reefs and in the same water. The fourfold figure is a ratio of risks ^[7], and a ratio needs a baseline before it becomes a count of dead coral. The phys.org account does not report how many colonies were tracked, what share of healthy coral died, whether the comparison was adjusted for species, or whether water quality was measured at the sites.

Disease was not assigned at random, so the ratio is an association. Colonies that already carried lesions may also sit in dirtier water or belong to more vulnerable species. Earlier studies found certain corals were hit particularly hard by disease and hot water together ^[17]. James Porter, a co-author and Josiah Meigs Distinguished Professor Emeritus in UGA's Odum School of Ecology ^[18], spoke about the elkhorn die-off. "The loss of this coral throughout the Caribbean is comparable to the death of oak trees from Maine to Florida," he said ^[15]. The neighbour result is just as ambiguous. Pathogens spreading from a lesion would produce it. So would neighbouring tissue sharing one poor patch of water.

The policy case rests on two links. The first is that corals in poor water frequently carry more disease, and more severe disease, than corals in cleaner water ^[13]. The coverage presents that as general background. The study supplies the second link, from disease to death in the heat. "Water quality protection is a local issue," Porter said. "These results demonstrate that measures to improve local water quality will also improve coral survival." ^[20] I think the direction is right. Cutting coastal pollution can reduce the pathogenic bacteria and nutrient loads that stress corals and can cause disease ^[16]. How much survival improves on Keys reefs depends on how strongly local water drives lesions there.

Camilla Nivison, the corresponding author and a doctoral student in the Odum School ^[19], made the restoration case directly. "We should be focusing restoration efforts in places where the corals are most likely to survive," she said ^[5]. For edges, she offered a reason. "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." ^[2] The explanation is not settled. "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," she said ^[3]. The survival data come from colonies already growing at reef edges ^[1]. Planted fragments are a different population, and whether they get the same advantage at an edge is a separate experiment.

Every summer since 2023 has brought similarly hot, prolonged temperatures ^[21]. "We've been monitoring coral reefs for a very long time, and they are in trouble," Porter said. "Since I began diving in 1972, we've lost more than two-thirds of all living coral worldwide." ^[22]