Eastern Mediterranean turning tropical, as warming waters force native species out

Temperatures in the Eastern Mediterranean and the Gulf of Eilat are reaching near-record highs this year and peaking much earlier than in previous years, experts say, with the former slowly turning tropical.

Both areas were already warmer than average at the end of winter in March, and with summer lasting through to September could still break local records.

Global average sea surface temperatures reached an unprecedented 20.96°C (just under 70°F ) on 31 July, according to the EU’s Copernicus Climate Change Service. Through June and July, marine heatwaves hit regions from the northeastern Atlantic and the Baltic Sea to the waters around Florida.

Across much of the Mediterranean, according to Copernicus, surface sea temperature anomalies (departures from average conditions) were as much as 3°C, (5.4°F), reaching as high as 5.5°C (9.9°F) along the coasts of Italy, Greece and North Africa.

In Israel, national monitoring stations are located at Hadera in central Israel, Ashkelon on the southern Mediterranean coast, and Eilat on the Red Sea at the country’s southernmost tip.

In Hadera, the highest daily sea surface temperature this year was 30.4°C (86.7°F) recorded on July 24, according to Tal Ozer, who studies the southeastern Mediterranean and coordinates data for the national marine monitoring program at Israel Oceanographic and Limnological Research institute. This was around one degree higher than the average for the past 12 years — the period during which comparable data of the current quality has been collected. It was just half a degree below the maximum ever recorded — 30.9°C (87.6°F) on August 16, 2021.

Ozer explained that the Mediterranean Sea, being relatively small and with a limited connection to the Atlantic Ocean at its western extreme, reflects and amplifies global trends. The southeastern part, which borders Israel, has displayed the highest temperatures and salinity, as water from the Atlantic Ocean has ample time to heat up as it covers the distance.

What was notable was not only that temperatures were climbing, but that they were reaching such high levels much earlier in the year, Ozer went on.

“We would normally expect the temperature to peak from mid-August to the start of September,” Ozer said. “There were years, like 2012 and 2016, during which we hit 30.8°C (87.4°F) in August. Now, it seems, we’re hitting these high temperatures by the end of July, and we’re not even near the end of the hot period.”

Ozer stressed that temperatures at the end of winter had also been unseasonably high. In March, the surface temperature in Hadera never dipped below 18.6°C (65.5°F). That was 1.2°C (2°F) higher than the average for the past 12 years.

In past years, sea surface temperatures measured off the coast of Ashkelon have been 0.3-0.4°C (0.5-0.7°F) hotter than those in Hadera, which is further north.  Real-time data has been lacking since March when a crane collapsed at the Rutenberg power station, damaging the monitoring station that is located there.

In Eilat, Yoni Shaked, who gathers and analyzes marine data for the national marine monitoring program, told a similar story to Ozer’s. Water surface temperatures in March were relatively warm — 18.6°C (65.5°F) was recorded on March 20 — and during summer the water was getting hotter.

The highest surface sea temperature this year in the Gulf of Eilat was 30.5°C (86.9°F) on July 28, he said — just under the maximum measurements taken in August 2020 and 2021.

The topography of the Gulf of Eilat helped to explain why its waters were heating at a relatively fast pace, Shaked said. The Straits of Tiran — a narrow landmass that connects the Gulf with the Red Sea to the south — allow only surface water to flow north, and surface water is always hotter than the deep sea.

Prof. Gil Rilov, a scientist at the National Institute of Oceanography, part of the Israel Oceanographic and Limnological Research institute, and a lecturer at Haifa University said, “The Eastern Mediterranean is changing from a temperate Atlantic ecosystem to a Tropical Indo-Pacific one.”

The southeast Levantine basin where Israel is located is “the hottest hotspot of change,” he added, a region that has become borderline comfortable for native marine species, many of which have disappeared, probably because of the fast-warming waters in the region. Many natives have been replaced by alien species, mostly from the Red Sea, which is connected to the Indian Ocean via Bab-el-Mandeb and the Gulf of Aden.

Rilov said more than 500 species had invaded from the Red Sea to the Eastern Mediterranean, either directly via the Suez Canal or in the ballast water of ships traveling through it.

“There’s a decade of research showing native biodiversity collapse in the Eastern Mediterranean,” he said. Most of the species hit were invertebrates, he added, and some played important roles in their ecosystems.

One key predatory snail, a whelk, had almost disappeared from the Eastern Mediterranean, he said, along with some once highly abundant sea urchins.

“At the same time, 60 to 100 alien species have been invading every five years since the late 1980s. Around 45% of the fish on rocky reefs are now alien species, along with 95% to 99% of bivalves and snails (mollusks).”

Invasive species making their homes in the Eastern Mediterranean that are considered hazards worldwide include the lionfish, a ravenous predator native to the Red Sea, and the rabbitfish which decimates algae, he said.

According to Rilov’s research, most native species that have been tested are resilient only up to 25-29°C (77-84.2°F), compared with alien tropical ones that can survive well up to 32-35°C (89.6-95°F) and even higher. Furthermore, while some important native seaweeds that create underwater forests are abundant in spring, alien seaweeds have high cover all year round because they can handle or even thrive in the hot summer temperatures of today.

As the planet warms and the icebergs at the poles melt, sea levels are also rising.

Rilov cited a 15-centimeter (six-inch) rise in the Eastern Mediterranean since the 1990s, although he said the figures were hotly contested.

Even the most conservative models predict a further rise of 50-60cm (20-24 inches) or even more by the end of the century, he went on. “Even with a 50cm rise, coastal ecosystems — including the highly vulnerable and unique vermetid reefs that reside along the Israeli coast — will disappear,” he said. Vermetid reefs are formed by small to medium-sized sea snails.

Prof Maoz Fine of Jerusalem’s Hebrew University and the Eilat Interuniversity Institute for Marine Sciences said that Israel’s corals were safe at the moment and had proved resilient up to 32°C (89.6°F).

But he added, “In combination with more local stressors, our corals won’t be safe for much longer.”

Unless pollution from multiple sources such as crude oil, agricultural chemicals, sewage and mariculture was reigned in, “we will see coral bleaching, and because the Gulf is a relatively closed basin, the damage will last for many years,” he said.

As for the resilience of fish, a paper published in 2020 by Prof. Amatzia Genin — Emeritus Professor of Marine Ecology at Jerusalem’s Hebrew University and the founder and former scientific director of the Gulf of Eilat national monitoring program — proposed that the speed of onset of a marine heatwave, rather than its peak, was likely what triggered a mass die-off of coral fish in the Gulf of Eilat in July 2017.

During that month, there were two instances of sudden seawater heating — in one, temperatures rocketed by 4.2°C (7.6°F) in just two and a half days. Two weeks later, they shot up by 3.4°C (6.1°F), also over two and half days. In both cases, temperatures remained at over 28°C (82.4°F) for two or three days.

Hundreds of heat-stressed fish, belonging to dozens of species, became
fatally infected with a pathogen that had evidently become more virulent with the sudden rise in water temperature.

“Dead fish were floating everywhere,” Fine recalled. “And many of them were  eaten by marine scavengers, which got infected as well.”

Two weeks ago, Fine inaugurated a new monitoring station 42 meters (154 feet) below the water’s surface.

“Even at that depth, the temperature is exceeding 27°C (80.6°F), which is crazy,” Fine said.

Noting that this was an El Nino year, which is likely to strengthen warming trends, he said he had not yet noticed changes in marine species off the coast of Eilat.

“But I’m sure there are changes in micro-organisms and species that we can’t see,” he added. “We know and see so little of what’s down there.”