As summer warms Istanbul's waters, the megacity's authorities are having to battle a noxious algal bloom dubbed "sea snot".

On a crisp, bright late April day, when the cold poyraz winds blow down from the Black Sea in the north, Koenraad Marinus van Lier contemplates the sea. He lives on Burgazada, an Istanbul islands where cars are forbidden, and the Sea of Marmara wraps around every corner like a big blue hug.

Van Lier is an artist. He is also a swimmer. Every sharp winter morning on the island, he swims in the sea just downhill from his studio, taking long strokes in the cold water. In the summer, the island fills up with weekend tourists and summer house residents, but during the off season, the sound of birds chirping and the soft wind in the fragrant pines comes through clear and strong.

Recently, though, van Lier had to stop swimming. As the water and the weather warmed, a familiar plague returned to the Sea of Marmara. It's early in the season, but van Lier has already seen the mucilage.

The Sea of Marmara is the world's smallest sea. It is also densely settled, highly industrialised, and semi-enclosed. "The Marmara Sea is like a bathtub," van Lier notes. There are only two narrow entry points: the Bosphorus Strait, which leads to the Black Sea, and the Dardanelles Strait, which leads to the Aegean Sea. This makes the sea particularly vulnerable to marine mucilage, also colloquially known as sea snot.

In the early summer of 2021, a plague of mucilage struck the Sea of Marmara. The gunky, sticky ropes of algae hovered like a bad dream on the surface of the water, strangling fish and marine life, leaving a film of bacteria behind. The mucilage is an overgrowth of phytoplankton, which coats the living things in the water with a mucus-like layer of slime that prevent oxygen transfer and can kill fish larvae, eggs, mussels, corals and whatever else comes in its path. The global news leaped on the story and measures were taken to reduce the mucilage. By June 2021, the problem was declared under control. But the mucilage never fully went away. It's been deep under the surface, mucking things up far out of sight of 18 million people who call Istanbul home. And now, it is returning to the surface.

"Mucilage is essentially an ecological situation, an ecological disaster… but the situation we have experienced in the Sea of ​​Marmara in recent years is not natural," says Mustafa Sarı, a professor in the department of water resources management at Bandirma Onyedi̇ Eylül University. According to Sarı, three factors coming together trigger the mucilage.

The first is climate. The world is getting hotter and so is the Sea of Marmara. "The Sea of Marmara is currently 2.5C warmer than the average temperature data of 40 years. The temperature is high," says Sarı. "In other words, it is related to climate and is [in the short term] beyond our control, we cannot control it. I wish we could."

The second is the natural state of the Sea of Marmara. The water that comes in from the Black Sea in the north is low in salt, while the water from the Mediterranean in the south is very salty. The difference in salinity and density means that the water from the two seas has trouble mixing together, creating a transition layer in the Sea of Marmara that limits the vertical circulation of the water. This can trap the mucilage under the top layer. "For this reason, the Sea of ​​Marmara is an ideal environment for ecological events such as mucilage formation," says Sarı. "This is also out of our control."

The third factor, however, is human-made. There are more than

© BBC