Cloudy bathing fun

Croatia’s “sea snot”: Just disgusting or dangerous?

On the right the Adriatic four weeks ago, on the left the brown soup from last weekend.(Bild: Krone KREATIV/Ekelstrand: zVg Schöner Strand: Leserreporter)

Bright sunshine, sweltering heat, crowds of tourists on the beaches - and no one who wants to cool off in the Adriatic. What is behind the "sea snot" that is currently turning Croatia's coasts into deserts of slime? And is this natural phenomenon just disgusting or even dangerous for swimmers?

"I'm really tough, water temperatures of around 16 degrees are no problem and even jellyfish don't normally bother me - but even I didn't want to get into the water!" For "Krone" reader reporter T., the short trip to Poreč last weekend turned into a nightmare, at least in terms of swimming. Brown slime everywhere, blocking the entrance to the otherwise turquoise-blue waters. "We walked along the coast for an hour looking for a place with less slime - to no avail."

The phenomenon that plagues Croatia's coast is officially and charmingly known as "sea bloom". It is also known as "sea snot" on social media - and that is certainly more appropriate. 

The slimy mass is the excretory product of certain algae. Sooner or later, it settles on the seabed. According to experts, the algae multiply due to higher temperatures, untreated wastewater that is discharged directly into the sea and low flow velocity.

The "snot" kills ecosystems
In fact, the phenomenon is not a new one, but has plagued the Istrian coast year after year - although usually only at the end of June and beginning of July. According to experts, it is mainly triggered by a sharp rise in sea temperature.

The reader reporter also noticed this: "I was in Pula four weeks ago and the water was less than 20 degrees - now it was at bathtub temperature." A look at the measuring stations shows that while it was a cool 20 degrees a week ago, it is now just under 24.

So how does the unsightly "snot" come about? Because phytoplankton multiplies quickly when there are (too) many nutrients available at high temperatures. This results in slimy accumulations of single-celled algae in the water column and on the surface of the water. Their growth is particularly strong when there is high air pressure and calm seas for longer periods.

The "marine bloom" can also destroy entire ecosystems when the algae sink to the sea floor, where their organic matter decomposes. This consumes oxygen and leads to a lack of oxygen in the water. This in turn causes organisms to die, which then decompose, depriving the water of oxygen and triggering a chain reaction. This was particularly dramatic in the Turkish Sea of Marmara in 2021, where hoses were even used to try and suck out the slime in order to save the ecosystem there. 

Incidentally, it is not only climate change that is to blame for the "sea snot"; humans are exacerbating the effect, for example by allowing too much nutrient-rich wastewater from agriculture to flow into the seawater.

Can you swim in the slime?
Anyone who dares to jump into the water despite the slime is well advised to take a good shower afterwards. The slime is not normally harmful to health, at least in the Adriatic, as it is made up of native and non-toxic algae species. However, according to the test by reader reporter T., a slimy film remains on the skin.

And: In the western Mediterranean, too, there have already been reports of a "harmful marine bloom" for human health caused by introduced algae species.