Warming Seas, Dying Reefs: Coral Bleaching and Disease Along Florida's Atlantic Coast

Florida's Coral Reef is the only barrier reef in the continental United States, running roughly 350 miles along the Atlantic side of the state, from the Dry Tortugas up through the Florida Keys and along the southeast coast past Miami-Dade, Broward, and Palm Beach counties. In the summer of 2023 it experienced the worst mass bleaching event ever recorded there, and since 2014 it has also been fighting a separate, unrelated disease that is killing coral even in years without a heatwave. Both problems trace back to the same root cause: a warming ocean is pushing a 25-million-year-old symbiosis past its limits.

A heat-driven breakdown of a partnership

Reef-building coral is an animal, but it gets most of its energy from a tenant living inside its own tissue: a single-celled dinoflagellate algae called zooxanthellae (genus Symbiodinium). The algae photosynthesize and share the sugars they produce with the coral, which in return gives the algae shelter and access to sunlight — a textbook mutualism. In many corals this partnership supplies up to 90% of the coral's energy budget, which is why reefs can thrive in nutrient-poor tropical water that would otherwise support very little life.

When water gets too warm for too long, the algae's photosynthetic machinery is damaged and starts producing reactive, toxic byproducts (reactive oxygen species) instead of usable sugar. The coral responds by expelling its algae — sometimes digesting the damaged cells, sometimes ejecting them directly. Without the algae's pigments, the coral's white calcium carbonate skeleton shows through the now-clear tissue, which is the "bleached" look that gives the phenomenon its name. A bleached coral is not dead, but it has lost its main food source; if warm water persists, it starves, and it also becomes far more vulnerable to disease.

Florida experienced this directly in the summer of 2023: an unusually intense marine heatwave, amplified by El Niño on top of long-term warming, arrived about a month earlier than the typical August–September peak and pushed reef temperatures as high as 93°F (about 5°F above the historical average, with some shallow patch reefs exceeding 38°C). Bleaching prevalence approached 100% on parts of Florida's Coral Reef, the most severe event on record.

A second, separate killer: disease

Since 2014, coral along southeast Florida has also been dying from stony coral tissue loss disease (SCTLD), first observed near Miami-Dade and Broward counties and since spread north to Martin County and south and west through the Keys to Key West. SCTLD is suspected to be caused by a bacterial pathogen, is unusually fast-moving, and has affected at least 22 of the reef's 45 coral species — some, like pillar coral, can die within weeks of infection. Between 2015 and 2018 alone, the outbreak destroyed an estimated 59% of live coral tissue in the affected region. Bleaching and SCTLD are different diseases with different immediate causes, but warmer, more stressed water makes coral more susceptible to both at once, so Florida's reef is often fighting on two fronts simultaneously.

Why losing coral reshapes the whole ecosystem

A coral reef's three-dimensional structure — its branches, plates, and crevices — is what makes it valuable habitat, not just its living tissue. That structure functions as a limiting factor on reef fish populations: it provides shelter from predators, especially for small and juvenile fish. When coral dies, wave action and erosion flatten the reef's complexity within a few years, and that directly lowers the reef's carrying capacity for fish. Research on this relationship has found that losing structural complexity can cut fishery productivity by more than half, sometimes by a factor of three, because fewer fish survive to larger, more productive size classes without places to hide. In ecological terms, the coral itself is a foundation species: its decline doesn't just remove one organism, it removes the physical habitat and the base of energy flow that an entire community of fish, invertebrates, and other reef life depends on.

Natural selection in real time — but is it punctuated equilibrium?

Not all corals bleach equally. Within the same reef, some colonies carry genetic variants (in the coral host, in its algal symbionts, or both) that tolerate heat better, and these survivors are more likely to live and reproduce after a heatwave. That is natural selection acting on existing variation within a population — classic microevolution, playing out over just a few bleaching events rather than over geologic time. Scientists are now trying to speed this process up deliberately through assisted evolution: selectively breeding coral from the hottest, most extreme reefs with coral from cooler reefs (one study found this raised offspring heat survival by as much as 84%), and testing whether swapping in more heat-tolerant strains of Symbiodinium can buy coral more thermal headroom.

It's tempting to call this "punctuated equilibrium," but that term describes something more specific: a pattern in the fossil record where a species shows long periods of little visible change (stasis) punctuated by relatively brief bursts of rapid change tied to speciation — usually in a small, isolated population. What's happening on Florida's reef right now is different: it's ordinary natural selection acting quickly on standing genetic variation within a single species, in response to unusually strong and sudden selection pressure (extreme heat). No new species has formed, and there's no isolated population undergoing rapid divergence. A precise answer distinguishes the pattern (punctuated equilibrium vs. gradualism, which describe the pace of change visible across generations or in fossils) from the mechanism (natural selection, mutation, genetic drift, gene flow, which explain why change happens at all). Rapid heat adaptation in coral is a mechanism story, not evidence of the punctuated-equilibrium pattern — and whether that selection can happen fast enough to keep pace with the rate of ocean warming is exactly the open question researchers are racing to answer.

Where this shows up on the Biology SOL

This single scenario touches two full strands of the Virginia Biology SOL, and knowing how to sort its details into the right bucket is exactly the skill a high-stakes multiple-choice item is testing.

BIO.8 — Ecology (populations, communities, ecosystems)

Carrying capacity and limiting factors (reef structure limiting fish populations), energy flow through an ecosystem (zooxanthellae photosynthesis feeding the reef food web), and the effects of natural events and human activity on an ecosystem (a warming, human-driven climate stressing a natural system) all appear directly in this reading.

BIO.7 — Evolutionary biology (how populations change over time)

Genetic variation and environmental pressure affecting survival, natural selection producing adaptations, and — the classic trap question — correctly distinguishing the *pattern* of evolutionary change (gradualism vs. punctuated equilibrium) from the *mechanism* driving it (natural selection, mutation, genetic drift, gene flow).

Test-taking note

SOL items often hand you a real scenario and ask you to pick the one correct vocabulary term. A perfect score depends on definitional precision: knowing that "carrying capacity" is a population's maximum sustainable size while a "limiting factor" is what sets that ceiling, and that "punctuated equilibrium" names a fossil-record pattern of speciation, not any instance of fast adaptation.

Zooxanthellae (Symbiodinium)

Photosynthetic dinoflagellate algae that live inside coral tissue in a mutualistic relationship, supplying the coral with most of its energy.

Coral bleaching

The loss of a coral's symbiotic algae (and their pigments) under thermal stress, leaving the coral's white skeleton visible through clear tissue.

Carrying capacity

The maximum population size an environment can sustain long-term, given its available resources and habitat.

Limiting factor

Any resource or condition (such as shelter, food, or space) that restricts the growth of a population.

Natural selection

The process by which individuals with heritable traits better suited to their environment survive and reproduce at higher rates, changing the traits common in a population over generations.

Punctuated equilibrium

A pattern observed in the fossil record in which species show long periods of little change (stasis) interrupted by relatively brief bursts of rapid change associated with speciation, contrasted with gradualism (slow, continuous change).

Check your understanding

Background sources to verify and cite before publishing: NOAA Florida Keys National Marine Sanctuary coral bleaching FAQs and disease outbreak pages (floridakeys.noaa.gov); Florida Fish and Wildlife Conservation Commission coral bleaching updates (myfwc.com); "Too Hot to Handle? The Impact of the 2023 Marine Heatwave on Florida Keys Coral," Frontiers in Marine Science (2024); peer-reviewed research on stony coral tissue loss disease outbreaks in southeast Florida (PLOS One, Frontiers in Microbiology); research on coral assisted evolution and heritable heat tolerance (Current Biology; AIMS Assisted Evolution program); research on habitat structural complexity and reef fishery productivity (Current Biology, "Habitat Complexity: Coral Structural Loss Leads to Fisheries Declines"); 2018 Virginia Science Standards of Learning Curriculum Framework, Biology strand (doe.virginia.gov).