What does extreme heat mean for the Mediterranean Sea?

July temperatures in the Mediterranean reached record levels. Frequent heat waves cause huge damage to underwater ecosystems and researchers expect more of them in the future. What are the solutions to heat stress?

Extreme heat has plagued the Mediterranean for weeks. Wildfires raged across at least nine countries in the region from Algeria to Greece. But the soaring temperatures are not only a danger for people and ecosystems on land, they're also harming marine life.

At the end of July, Mediterranean Sea surface temperatures hit a record 28.7 degrees Celsius (83.66 Fahrenheit), with some eastern parts of the waters reaching more than 30 C. Those temperatures could rise further in August, which is usually hotter.

"Without a doubt global climate change is the main reason for the heat waves in sea. It's causing the ocean to warm," said Katrin Schroeder, an oceanographer with the Institute of Marine Science in Italy. 

In a warming world, marine creatures are in danger of suffocating. Gases like oxygen and carbon dioxide dissolve better at colder temperatures, so that means the warmer the water, the less oxygen is available to breathe.

Conversely, higher temperatures also cause an increase in metabolism, which in turn means animals have to breathe even more than usual, said Diego Kersting, a marine scientist with Spain's National Research Council (CSIC). That combination also heightens the risk of death by starvation for marine life.

"The rise in temperature accelerates metabolism, and the organisms need more food to maintain this metabolic rate," said Kersting.

High water temperatures are most harmful for animals living at the bottom of oceans, lakes or rivers. These benthic species include corals, mussels, sponges, starfish and plants like sea grasses, and are often attached to rock or solid ground. They can't migrate when it gets too hot.

Scientists observed mass deaths of benthic species along thousands of kilometers of Mediterranean coastline between 2015 and 2019.

"We had really complex ecosystems, rich in biodiversity, and now we're losing that, at least in shallow waters," said Joaquim Garrabou, senior scientist at CSIC's Institute for Marine Sciences in Spain.

Many benthic species are crucial to the marine ecosystem. They filter the water and keep seas, rivers and lakes clean by eating dead organisms. Some species are an important food source for other creatures or are harvested by humans. Benthics like soft corals, seaweed and seagrasses provide some of the main ocean habitats.

Heat is particularly harmful for Posidonia oceanica or Neptune grass, said Pedro Beca-Carretero from Germany's Leibniz Center for Tropical Marine Research (ZMT). And the large, slow-growing seagrass is found only in the Mediterranean. Previous heat waves have decimated the species, which is bad news for the climate.

"This species is of particular importance to humans as it serves as a major natural carbon sink and stores more carbon per square meter than forest ecosystems, making it one of the most effective ecosystems for long-term carbon storage," said Beca-Carretero.

Jellyfish, on the other hand, are thriving because of higher temperatures, as well as nutrient run-off from farms and sewage. Overfishing and loss of fish habitat mean the jellyfish have few or no predators. When currents push the animals together, the Mediterranean turns into a crowded jellyfish hotspot.

The sea also hosts around 1,000 invasive species — the highest number in the world. While this is not directly linked to climate change or rising temperatures "such conditions clearly favor species introduced from warmer seas," said Kersting from Spain's CSIC.

Alien species can have a major impact on ecosystems. For instance, invasive Rabbitfish native to the Indo-Pacific and Rea Sea feed on seaweed and have reshaped the habitat of the eastern Mediterranean. Underwater deserts have replaced dense seaweed forests.

"The species that normally live in the algal forests — and the species that usually feed on them — can no longer live there," explained Joaquim Garrabou of the CSIC.

Warming seas are already affecting fishing activities in the area. Fishermen are catching fewer familiar species and instead are finding more invasive fish which they have difficulty selling.

 "Rabbitfish and lionfish are edible, but other invasive fish aren't. Some are even poisonous, like the puffer fish," Garrabou said.

Habitat loss could also lead to an overall decline in fish populations, while disappearing seagrass means coasts will be more exposed to future storms. This could also have a knock-on effect for tourism because divers will be less likely to visit an impoverished underwater landscape.

One thing all the researchers agree on is that to save the Mediterranean Sea habitat, humans must stop emitting greenhouse gases.

"There's no way to directly protect the habitats of the Mediterranean Sea against heat stress, but they can be made more resilient," said Christian Wild, head of the marine ecology department at the University of Bremen in northern Germany.

One important step in doing so is combatting the growth of algal blooms which are worsened by run-off from agriculture, wastewater and industry, added the scientist.

Scientists also hope that the UN goal of protecting 30 per cent of the world's oceans by 2030 will directly benefit the Mediterranean. So far, just 8 per cent of the sea is protected.

"Above all, we need to increase the number of strictly protected areas where fishing, diving and boating are not allowed," said Joaquim Garrabou. Even if marine protected areas won't make the water cooler, "we are seeing that well protected areas are recovering faster and better from human disturbance," he added. 

Designating protected areas isn't enough, they also have to be properly managed, which is something that's lacking now, said the researchers.

Some of the Mediterranean's new inhabitants could also be a helpful addition as the planet heats up.

Tropical seaweed Halophila stipulacea Ascherson, originally native to the Red Sea, copes well with rising temperatures and salinity levels compared to other seaweeds.

It's an invasive species that could potentially help "seagrass beds survive in a smaller part of the Mediterranean and continue to provide some of their essential ecosystem services," said Pedro Beca-Carretero.

There may also be hope for the native Neptune grass. Studies show that the plants can deal with rising Mediterranean temperatures if they are intentionally exposed to heat as young seedlings.