On a summer evening in July 2023, a monitoring buoy in a shallow Florida bay registered a number that scientists first assumed was a sensor glitch.
It was not a glitch.
The reading climbed past 100 degrees and kept going, settling at 101.1 degrees Fahrenheit, water as hot as a household hot tub, sitting inside a bay tucked between the southern tip of Miami-Dade County and Key Largo.
The bay has a name most Americans would recognize in an instant, and what that name means for the story ahead is something almost no one was expecting.
The number that stopped scientists mid-sentence
A water quality station in Manatee Bay in Everglades National Park measured a temperature of 101.1 degrees Fahrenheit at 6 p.m. at a depth of just 4.9 feet.
At first, it was assumed the reading was a sensor error.
But when those watching checked the neighboring buoys, they were stunned to find temperatures of 99.3 degrees at Murray Key and 98.4 degrees at Johnson Key.
The numbers were real, and they were everywhere.
Temperatures remained high even after sunset, with readings up to 99 degrees at 10 p.m.
“These waters were some of the hottest ever recorded on Earth,” said NOAA hurricane scientist Dr. Jeff Masters.
Scientists scrambled to make sense of what they were seeing, because the full consequences of water this warm reach far beyond a single buoy reading.
Why shallow bays cook so fast
Manatee Bay is not the open Atlantic.
The buoys inside Florida Bay sit in very shallow, murky, dark water, and because the water is contaminated with sediment, darker surfaces absorb more heat.
Mud flats, mangroves, and submerged aquatic vegetation more readily absorb sunlight, which can account for the very high readings.
Climatologists have attributed the warm waters to the El Niño climate pattern and an ongoing marine heat wave, with sea surface temperatures at least 2 degrees above average.
Relative to temperatures recorded from 1991 to 2020, the waters surrounding South Florida were 5 to 7 degrees Fahrenheit above normal.
A few degrees of difference in a shallow bay does not stay local for long, radiating outward through the seagrass beds, into the reef, and up through the food chain.
What happens to a reef when the ocean becomes a hot tub
Florida’s coral reef is the largest in the continental United States.
It stretches around 350 miles from the Dry Tortugas National Park to St. Lucie Inlet in Martin County.
Unprecedentedly high ocean temperatures place corals at severe risk of bleaching, as heat stress causes coral hosts to expel the colorful microscopic photosynthetic algal cells from their tissues.
Researchers diving at a coral reef restoration site off South Florida found 100% coral mortality.
When coral is harmed by warm ocean temperatures, it can have a “cascading effect” on the other marine life that depends on it.
But the creature the bay is named for has its own complicated relationship with warm water, one that makes this particular reading almost unbearably ironic.
The animal that loves warm water, pushed past its limit
Manatees are famous for chasing warmth.
Every winter, thousands of Florida manatees crowd around power plant outflows and natural springs, because water below 68 degrees Fahrenheit can kill them.
The bay bearing their name was, for a long time, one of the richest seagrass habitats in South Florida, and seagrass is what manatees eat, almost exclusively.
Elevated temperatures can exacerbate coral bleaching, hypoxia, and other issues forecast to intensify with the progression of global warming.
Hypoxia, low oxygen in the water, suffocates the seagrass beds that manatees depend on for every meal.
“High temperatures over long periods of time can have a detrimental impact on the marine plants and animals,” said Allyson Gantt, chief of communications and public affairs for Everglades and Dry Tortugas National Parks.
The animal that needs warmth to live has now met a version of warmth that strips away its food, its shelter and its reef.
A bay worth fighting for
“Without a structurally sound reef, southeast Florida can suffer ramifications immediately, as the first natural barrier to major wave movement from storms becomes less effective,” said Florida Sea Grant agent Ana Zangroniz.
Coral restoration crews are already replanting and monitoring what remains.
The good news is that temperatures can still decrease by as much as 10 degrees at night.
Salinity levels in this area are also lower at this time of year than in many previous years, offering the ecosystem some small relief.
Scientists note that the 101.1-degree reading came from an unusually shallow, sediment-rich location, and not every coastal habitat is warming at the same rate.
While the potentially record-breaking temperature was temporary, the current above-normal temperatures are part of ongoing unprecedented extremes, and scientists expect more records to fall as conditions continue to evolve.
The manatee has survived ice ages, hunting and habitat loss.
The bay named for it is now a living thermometer, and scientists are watching it more closely than ever, hopeful that the creature and the reef beneath it prove just as stubborn as their track record suggests.