A Slimy Calamity Is Creeping Across the Sea

The sea snot blanketing Turkey’s coastline isn’t just gross—it’s also smothering animals underwater.

A crab entangled in sea snot
Sebnem Coskun / Anadolu Agency via Getty

Divers who have seen the phenomenon firsthand describe many types of underwater sea snot. There are the “stringers,” which most resemble the sticky goo that might actually come out of your nose. But there are also floating “clouds,” white and ethereal, so delicate that they break apart in your fingers. Then there are the tiny flakes of “marine snow,” which begin as drops of mucus and accumulate organic debris as they drift slowly, slowly down to the bottom of the sea.

Then there is whatever is happening off the coast of Turkey—a downright “mucilage calamity,” in the words of Turkish President Recep Tayyip Erdoǧan. The sea snot there has surfaced and turned monstrous, gelling into a thick layer of yellowing slime atop the water. For months, this foul mucus has blanketed the Sea of Marmara, which connects the Black Sea and the Aegean Sea in the Mediterranean. It’s smothering shellfish, clogging nets, and destroying the fishing industry.

The slime is, in short, a national crisis. Turkey is now trying to vacuum up its embarrassment of sea snot, dispatching workers with hoses to collect mucus by the tons for incineration. But scientists say that much more is probably lurking under the water. And even worse, the floating mucus is a sign of much larger disruptions in the sea. As unsightly as sea snot might be, its most devastating effects happen far away from human eyes, deep below the surface.

Aerial photo of sea snot
An aerial shot of sea snot covering the Sea of Marmara (Umit Bektas / Reuters)
Workers vacuuming up sea snot
Workers in Turkey vacuuming up sea snot (Ali Atmaca / Anadolu Agency via Getty)

Slime in the sea is not inherently unusual. “Mucus is everywhere,” says Michael Stachowitsch, a marine ecologist at the University of Vienna. “There’s no marine organism that doesn’t produce mucus, from the lowly snail to the slimy fish.” But in healthy waters, mucus doesn’t amass to epic proportions. The current sea-snot outbreak can be blamed on phytoplankton, a type of algae that produces the small bits of mucus that turn into flakes of marine snow. When these phytoplankton receive an infusion of imbalanced nutrients from fertilizer runoff or untreated wastewater, they make an overabundance of mucus. Beads of that mucus accumulate into stringers, which accumulate into clouds, which accumulate into the unending sheets now washing up on Turkey’s coast.

But pollution alone doesn’t explain the appearance of so much sea snot—or marine mucilage, to use the scientific term. This much slime buildup also requires specific weather conditions: hot and calm. In spring and summer, the sun heats up the top layer of seawater, leaving a layer of cool, denser water underneath. (Salinity also plays a role in the density gradient: Saltier water will sink beneath fresher water.) Because of this gradient, the mucus will sink until it starts to float; then it lingers. The longer it stays, the more it accumulates. And without strong winds or storms, nothing creates turbulence to churn the water and rip the mucus apart.

Bacteria trapped in the mucus will eventually start to eat and digest it, creating air bubbles that ultimately float the whole sheet of sea snot up to the surface. In the Adriatic Sea, the arm of the Mediterranean just east of the Italian peninsula, the floating mucus can dry and toughen in the sun. Seagulls are known to walk on it.

Mass outbreaks of sea snot have appeared dozens of times in the Adriatic over the past three centuries, probably because its geography and calm winds create the perfect conditions for large sheets to form. Sea snot has had big economic consequences there. “The main problems are fisheries and tourism,” Michele Giani, an oceanographer at the National Institute of Oceanography and Applied Geophysics, in Italy, told me. Boats cannot go to sea at all because mucus clogs up the seawater intake that cools the motor. “A motor can have a meltdown within a minute,” Stachowitsch said. Fishing nets become slimy and heavy. And tourists, of course, want nothing to do with the mess. It doesn’t help that as sea snot degrades on the surface, its smell can turn quite nasty too.

The first description of mare sporco, or “dirty sea,” in Italian dates back to 1729. But in the early 2000s, marine mucilage started breaking out pretty much every year, which scientists, in a 2009 paper, linked to climate change. (Huge swaths of marine mucilage have also turned up near Turkey at least once before, in 2007.) You might think of the snot as a symptom of “ocean flu,” says Antonio Pusceddu, a marine ecologist at the University of Cagliari, in Italy, who co-authored that paper: The snot’s appearance is a sign of deeper sickness in the sea, caused by climate change and pollution.

Diver with sea snot underwater
A diver surrounded by sea snot underwater in Turkey (Sebnem Coskun / Anadolu Agency via Getty)

The link between marine mucilage on the surface and the clouds and stringers underwater became clear during the 1980s, when researchers diving in the Adriatic first observed the unusual masses. Scientists had missed this phenomenon earlier, Stachowitsch said, “because the instruments that were used to bring up water samples from the ocean were quite brutal, so they shook up the water,” destroying the mucus. Humans could see it only if they went down themselves, either with scuba gear or in submersibles. Gerhard Herndl, an oceanographer now at the University of Vienna, told me that while diving in the ’80s, he mistook the first cloud of mucus he ever saw for a shark. Until that moment, he had not known that sea snot could grow to such behemoth proportions.

The mucus floating underwater was fascinating—even beautiful—but what scientists saw on the seafloor was disturbing. They already knew that unsightly layers of the mucus could float to the surface. Now they discovered that they could also sink, covering corals, sponges, brittle stars, mollusks, and any other unlucky creatures on the seafloor, cutting them off from oxygen. “They’re literally smothered,” says Alice Alldredge, an oceanographer at UC Santa Barbara. “Sure, it’s uncomfortable for us as human beings to have all this gunk at the surface. But the bottom-dwelling organisms are going to die.” An ecosystem takes years to fully recover from such a mass mortality.

For this reason, vacuuming up sea snot on the surface in Turkey is too little, too late for the local ocean ecosystem. To deal with the root of the problem, Turkey will have to prevent untreated wastewater and runoff from entering the Sea of Marmara in the first place. That certainly won’t clear up the sea this summer. But evidence from Italy suggests that such efforts might quell Turkey’s ocean flu in the long run. In the Adriatic, Pusceddu says, mucilage outbreaks have died down since Italy began cleaning up the wastewater that flows into it. The sea has returned to what looks like a healthier, less slimy normal.