In the scorching summer of 2023, Florida’s coral reefs faced one of the most severe bleaching events ever recorded, challenging scientists’ understanding of coral resilience and survival. Karen Neely, PhD, a research scientist at Nova Southeastern University (NSU), led a critical study examining how these vibrant underwater ecosystems responded to historic thermal stress.
“On land in the Florida Keys, heat records were shattered for 31 out of 62 days in July and August,” Neely recounts. Beneath the ocean’s surface, conditions were equally extreme. Scientists measure coral stress using a metric known as Disease Heating Weeks (DHWs), combining the intensity and duration of heat. Historically, since records began in 1985, Florida’s coral reefs rarely surpassed 10 DHWs. However, in 2023, coral reefs experienced an unprecedented 23 DHWs, dramatically amplifying the stress corals faced.
Coral bleaching results from disruptions in the symbiotic relationship between corals and the microscopic algae living within their tissues. These algae provide corals with up to 90% of their nutrition through photosynthesis. Neely vividly describes it: “It’s like having an apple tree growing inside you.” During prolonged heat stress, corals expel their algae, turning stark white as their skeletons become visible. Without their symbiotic algae, corals begin to starve and can eventually die if temperatures remain elevated.
Neely’s involvement in coral reef health monitoring began in 2019 during an outbreak of coral disease. Her team developed innovative in-water treatments to combat disease-related mortality, establishing a robust coral monitoring system across various reef locations. This system allowed Neely’s team a unique vantage point during the 2023 bleaching event.
Their meticulous monitoring of over 4,200 coral colonies — including key reef-building species such as brain and boulder corals — yielded both promising and concerning findings. Remarkably, mortality rates were relatively low at most monitored sites, ranging between 0 to 2 percent. However, brain corals fared significantly worse than boulder corals in the areas hardest hit by heat stress. This discovery highlighted critical differences in species resilience under identical thermal conditions.
Neely’s team also documented an unprecedented phenomenon: direct heat-induced coral mortality occurring rapidly, rather than resulting from prolonged starvation. “We noticed corals dying quickly in July, well before prolonged cumulative stress,” Neely explains. “It indicated direct heat stress mortality, something previously undocumented in the wild.”
Coral recovery from bleaching events depends significantly on subsequent environmental conditions. If ocean temperatures cool rapidly, algae may recolonize coral tissues relatively quickly, returning color and nutritional support. However, Neely emphasizes, even these recovered corals experience weakened resilience. Post-bleaching stress increases their susceptibility to diseases and drastically reduces their reproductive capabilities, impacting reef regeneration for years.
The repercussions of such bleaching events extend far beyond immediate coral health, potentially destabilizing entire ecosystems. Neely notes, “Corals are essentially the foundational bricks of the reef ecosystem. Without them, reefs erode, decreasing biodiversity and destabilizing habitats for marine life.” Reduced coral cover directly impacts Florida’s extensive “blue economy,” estimated to contribute over $800 billion annually to the state’s GDP. The consequences include diminished tourism, fisheries collapse and reduced natural coastal protection from storms and flooding.
The 2023 bleaching event also underscored vulnerabilities within ongoing coral restoration efforts. Corals frequently used in restoration projects suffered notably higher mortality rates, marking a significant setback. These observations prompt reconsideration of coral species selection and restoration strategies to enhance reef resilience in the face of climate change.
Recognizing these urgent challenges, Neely and her research team are developing new strategies for protecting corals from future bleaching events. During the summer of 2024, they experimented with deploying shade cloths over vulnerable coral colonies, successfully reducing heat-induced stress. While these initial results proved promising, ongoing research aims to assess whether such protective measures can provide longer-term resilience benefits.
Nevertheless, Neely stresses the critical need to address fundamental environmental factors: water quality and rising ocean temperatures. “Coral reef decline is often described as ‘death by a thousand cuts,’” she says. “Individual interventions are valuable, but systemic solutions addressing water quality and temperature are imperative for broader reef survival.”
Efforts are also underway to develop more resilient coral strains through selective breeding and stress-hardening techniques, which could strengthen future reef restoration outcomes. However, Neely cautions that these approaches alone are insufficient without broader, comprehensive climate action.
Community engagement plays a pivotal role in safeguarding Florida’s coral reefs. Neely advocates for increased public awareness and involvement, emphasizing that daily decisions significantly impact reef health. Simple actions such as reducing carbon footprints, choosing reef-safe sunscreens and practicing responsible boating and diving practices contribute meaningfully to reef conservation. Additionally, advocating for improved water quality and climate-smart policies remains crucial.
Public awareness of coral bleaching has noticeably increased, particularly following the stark visual impact and wide media coverage of the 2023 event. Neely describes the phenomenon as “undeniable,” noting how vividly communities experienced the effects of extreme heat firsthand. “The ocean became uncomfortable to swim in, and coral bleaching was visible everywhere,” she says. “People recognize these events as increasingly common and severe, creating an urgency for action.”
Despite the resilience demonstrated by many coral colonies, Neely acknowledges critical knowledge gaps in understanding coral tolerance thresholds to extreme temperatures. “We still don’t know precisely at what point temperatures become lethal for different corals,” she admits, underscoring the uncertainty facing future reef management.
Ultimately, Neely’s research provides both a sobering assessment of coral vulnerabilities and clear strategies for future conservation efforts. Immediate action on reducing ocean temperatures, improving water quality and innovating restoration techniques is paramount to preserving Florida’s precious coral ecosystems.
“We have actionable solutions,” Neely concludes optimistically yet firmly. “The critical challenge now is to ensure these solutions are broadly adopted before we reach a point where reefs can no longer recover. Every effort counts if we want to preserve these ecosystems for future generations.”
