Slovenian sea not immune to coral bleaching and tropicalisation

Matjaž Ličer, a physicist with the Environment Agency and the Marine Biology Station of the National Institute of Biology, points to a direct link between how climate change effects the atmosphere and the oceans.

The accumulation of greenhouse gases in the atmosphere is warming up the planet and most of this heat has so far accumulated in the oceans, over 90% of it.

Being shallow, the Mediterranean is warming up faster than the global average. "This is why the processes linked to climate change are developing faster and more intensively here," he says.

The surface layer of the north Adriatic has warmed up by about a degree Celsius over the past 30 years. "This does not seem much but you need to bear in mind that the upper ten metres of the ocean has the same heat capacity as the entire atmosphere.

"To warm up the surface layer of the ocean by one degree, you need enormous amounts of heat. This means this layer works as heat storage will keep warming up the planet for a long while, even if we manage to reduce emissions," Ličer explains.

Coral bleaching

Rising atmosphere temperatures, and consequently sea temperatures, have a major impact on marine life. Like elsewhere around the world, coral bleaching is a problem in the Slovenian part of the Adriatic Sea, especially in the summer months, says Lovrenc Lipej, a researcher at the Marine Biological Station.

"This is a process where symbiotic algae leave the coral, causing it to lose its colour. When the temperature drops again, the symbiotic relationship between the algae and the coral can be re-established. However, if high temperatures persist for a long time, more permanent damage and death of the polyps or the whole colony can occur," he explains.

Particularly vulnerable to this this process in the Slovenian sea is the cushion coral (Cladocora caespitosa), which is endemic to the Mediterranean and therefore unique to the area.

Coral bleaching is a particularly grave problem because corals are important habitats for many organisms, such as bristle worms, snails, mussels and others.

"We have found that a coral 70 centimetres in diameter can host more than 110 different species of organisms, which makes it an important habitat for several thousand organisms," Lipej says.

The Marine Biology Station, which is located in Piran, has been successfully growing corals in its labs, while attempts are still underway to reintroduce them into the sea itself.

"We are currently looking for areas with underwater springs where the temperatures are slightly cooler to test-populate the corals and see how they perform in this area," he says.

Species from warmer parts spotted in Slovenian sea

Apart from coral bleaching, rising temperatures are also attracting new species to our seas in a process known as tropicalisation. Lipej explains scientists use the term to describe a process where thermophilic species are spreading northward from the southern parts of the Mediterranean.

Quite a few interesting species mainly of fish that did not used to occur here in the past have been spotted in the Slovenian sea over the past 20 years. These include the grey triggerfish, black ruff, ocean sunfish, barracudas and the common dolphinfish.

An internationally recognised indicator of tropicalisation is the bluefish (Pomatomus saltator), which is often found in the estuaries of rivers.

"This species causes major problems particularly in Croatia. The bluefish is a successful predator that feeds on many other fish, including larger ones such as the mullet. It preys on large numbers of mullets particularly in the Neretva estuary. As it often occurs in large flocks, it also causes considerable damage to fisheries," Lipej explains.

Although the bluefish also occurs in smaller numbers in the Slovenian sea, it is not causing any major problems for now, according to Lipej.

Like most other thermophilic species, it only appears in the warm months, leaving in winter. The Slovenian sea is colder in winter compared to other parts of the Mediterranean due to its northernmost position in the Adriatic.

"The average temperature then is between 6 and 10 degrees Celsius, which is problematic for species that are typical of warm environments," Lipej explains.

However, he says the situation could quickly change in the future. If the sea keeps warming, this will mean smaller temperature differences throughout the year and an opportunity for thermophilic organisms to stay in the Slovenian sea longer.

Fear of lionfish exaggerated

Media attention has recently been grabbed by the news that a poisonous lionfish (Pterois miles) has been caught near the Croatian island of Korčula. This is an alien species, but it is not associated with tropicalisation, rather it has arrived via the Suez Canal.

The fish had been spotted in the Mediterranean several decades ago, and in 2019 for the first time in the Adriatic.

The invasion of its relative (Pterois volitans), which Lipey says was released from an aquarium in the 1980s by a reckless individual, is causing problems especially in the Caribbean Sea, where it predates on native fish and has poisonous spines that are also dangerous to swimmers.

"A similar scenario is feared in the Mediterranean. However, only a small number of lionfish have been spotted so far, this is not a major problem for now. It has not yet been spotted in the Slovenian sea either," Lipej points out.

According to the latest available report from 2021, 56 alien species had been detected in the Slovenian sea. Lipej says the figure has increased since, but remains among the lowest in the Mediterranean.

"Most such organisms appear in our sea only rarely, once or twice but then no more. Only a few have settled, and invasive species that cause environmental and economic damage account for less than 10% of all species."

The best known alien species in the Slovenian sea is the Japanese oyster, which can be found along the entire coast on almost every rock in the coastal belt.

The polychaete worms or the Mediterranean fanworms (Sabella spallanzanii) have also become established throughout Slovenian coastal wetlands.

It is an interesting species because it allows many native organisms to settle among its tubes, and is thus beneficial for marine biodiversity, Lipej points out.

The comb jelly (Ctenophora), a close relative of the jellyfish, which was initially described as one of the most problematic species because it is thought to feed on fish eggs and larvae, also appears regularly, especially in July.

However, a study at the Marine Biological Station, which examined the contents of 500 stomachs of the alien comb jellies, found they rarely contain fish eggs and larvae. "In the Slovenian part of the Adriatic, it is thus not harming fish populations and damaging fisheries," he says.

Slovenian sea rich in biodiversity

Despite certain changes to the biodiversity of the Slovenian sea, Lipej finds it is still good. "We have many species and regular dolphin populations, certain shark species are still present, the data on the coastal fish community is still of sufficient quality, and we have a low number of extinct species."

Fucus virsoides, a species of brown algae endemic to the Adriatic Sea, disappeared from the Slovenian sea in 2015, but since it still occurs in the marine environment of Savudrija, across the border in Croatia, there is some chance that it will return.

There is also a trend towards a decline of Cystoseira algae in the Mediterranean as a whole, including in the Slovenian sea. The Marine Biological Station is growing this type of brown algae in order to restore the population in the event of a mass extinction.

The station is also focusing on the study of the fan mussel as one of the most endangered species in the Slovenia sea. They grow them in the laboratory and are learning about their living conditions and the possibilities for their repopulation.

Amateur science helps in their research work. "Often, help comes from individuals who are not professional scientists but science enthusiasts. They often send us interesting photographs that may be of interest for our research," says Lipej, adding that they greatly appreciate any help.