There Is Only One Sea on Earth That Touches No Land, and Its Borders Don’t Truly Exist

Far out in the North Atlantic, roughly 590 miles east of Florida, a sprawling patch of water drifts in slow rotation. It touches no shore. Currents fence it in. For centuries, sailors crossed this region without realizing they had entered a distinct sea. Now forty years of data show that this boundaryless expanse is changing faster than almost anywhere else in the open ocean.

The Sargasso Sea has warmed by nearly one full degree Celsius since 1983 and grown more than 30 percent more acidic. That finding comes from an updated analysis published in Frontiers in Marine Science by researchers at the Bermuda Institute of Ocean Sciences, which Arizona State University now operates. The study draws on monthly measurements collected at the Bermuda Atlantic Time-series Study site, the longest continuous record of open-ocean carbon chemistry on Earth.

A Sea Bounded Only by Rotating Currents

No land encloses the Sargasso Sea. Four major currents create its borders: the North Atlantic Current, the Canary Current, the North Equatorial Current, and the Antilles Current. Together they form the North Atlantic Gyre, a clockwise swirl that spans roughly two million square miles.

The sea takes its name from Sargassum, a golden-brown seaweed that floats in thick mats across the surface. Portuguese sailors called it sargaço, after a grape-like algae they knew from home. The weed never anchors to the seafloor. It lives and dies adrift, and in doing so it builds a floating habitat that shelters juvenile loggerhead turtles, endemic shrimp, porcelain crabs, and dozens of fish species found nowhere else. The mats form an 800-mile-wide nursery where hatchling turtles hide until their shells harden and European eels begin life as translucent larvae before migrating to freshwater rivers.

That same isolation has made the Sargasso Sea an irreplaceable scientific sentinel. Since 1983, researchers have sailed to the BATS site each month to lower instruments and collect water samples from depths exceeding 4,200 meters. The dataset now spans four decades without interruption.

What Four Decades of Water Samples Reveal

The trends are stark and none show signs of slowing. Surface temperatures rose by 0.24 degrees Celsius per decade, adding up to 0.97 degrees since the early 1980s. Salinity climbed by 0.136 units. Dissolved oxygen fell by 12.5 micromoles per kilogram, a drop of about six percent. The water is warmer, saltier, and holds less breathable gas than it did when the measurements began.

Carbon chemistry shifted even more dramatically. Dissolved inorganic carbon increased by 51.5 micromoles per kilogram as the ocean absorbed carbon dioxide from fossil fuel emissions. Ocean pH dropped by roughly 0.075 to 0.1 units. That numerical decline translates to a greater than 30 percent rise in hydrogen ion concentration. The water is simply more corrosive now. Saturation states for aragonite and calcite, the minerals that corals and shell-forming plankton need to build their skeletons, have also decreased.

Nicholas Bates, a chemical oceanographer at Arizona State University’s Julie Ann Wrigley Global Futures Laboratory and lead author of the study, wrote in the Frontiers in Marine Science paper that ocean chemistry in the 2020s now falls “outside the seasonal range observed in the 1980s.” Conditions that once marked an extreme year now count as ordinary.

A Nursery Under Pressure

The Sargassum mats function as an open-ocean nursery. Hatchling turtles hide among the fronds. European and American eels begin life here as translucent larvae before swimming toward freshwater rivers where they will spend decades. Eventually they cross thousands of miles back to the Sargasso Sea to spawn once and die. Humpback whales pass through during spring migration. Tuna streak beneath the mats toward their own spawning grounds.

Those same rotating currents now funnel plastic waste into the gyre. One survey estimated 200,000 pieces of debris per square kilometer across hundreds of miles of open water. Cargo vessels slice through Sargassum mats, shredding habitat with their propellers. The low-frequency calls of sperm whales must compete with engine noise.

Researchers also warn that warmer layers of water resist vertical mixing, which starves deeper zones of oxygen and hoards nutrients that would normally rise to feed plankton blooms.

The Challenge of Protecting Waters No One Owns

No single nation holds jurisdiction over the Sargasso Sea. That legal reality complicates any effort to protect it. The Sargasso Sea Commission, an intergovernmental body formed in 2014 with support from the Government of Bermuda, works through existing international organizations to explore conservation measures. Options under discussion include rerouting shipping lanes to avoid dense Sargassum concentrations and restricting longline fishing during peak turtle migration.

Current projects funded by the Global Environment Facility and the French Facility for Global Environment aim to produce a socio-ecosystem diagnostic analysis of the region’s ecological and economic value. That assessment will inform a strategic action program that stakeholders can agree to implement.

The BATS and Hydrostation S data remain publicly available through the Biological and Chemical Oceanography Data Management Office. The researchers note that sustained observations like these will be essential for predicting how the North Atlantic and the life within it respond to conditions that continue to shift beyond historical bounds.

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