THESSALONIKI, Greece — Scientists have found a sharp oxygen shortage in the bottom waters of the Thermaikos Gulf, the shallow inlet beside Greece’s second-largest city, and warn that a much wider collapse in marine life could follow within weeks. On Friday, researchers said the affected area had reached about 430 square kilometres, with oxygen readings in some locations falling below levels many fish can tolerate.
A Thessaloniki-based marine science team recorded oxygen concentrations of 1.6 milligrams per litre at six sites this week, down from a late-summer average of 5 to 7 milligrams in the same waters. Readings below 2 milligrams are generally classed as hypoxic, a condition in which mobile species flee and less mobile animals can die. The team checked its instruments against water samples taken at 31 stations.
A gulf sealed from fresh air
The abrupt drop has surprised scientists because the gulf’s oxygen levels were not falling at this pace in August. Researchers suspect that unusually warm surface water has formed a stable lid over cooler depths, while the decay of a plankton bloom consumes oxygen below. They have not established why the change accelerated so quickly. “The water is spending oxygen faster than the gulf can replace it,” said Seija Suuronen-Tauriainen, a physical oceanographer on the team.
Officials say the readings do not indicate a threat to drinking water or public health. Gerald Lutz, a spokesman for the environment ministry, said the low-oxygen patches remained below the surface and were being mapped before any restrictions were announced. “There is no cause for people to avoid the coast,” he said, adding that the ministry would receive an updated survey on Monday.
The danger is not one dramatic moment; it is a shrinking patch of livable water, day after day.
Seija Suuronen-Tauriainen, physical oceanographer
For fishers in Nea Michaniona, a port on the gulf’s western shore, the change is already visible in the daily haul. Matías Prieto, a local skipper who has worked these waters for 18 years, said catches of sole and mullet had shifted toward the gulf entrance. “We used to find them close in. Now the nets come back light unless we go farther out,” he said. He has not been told to stop fishing.
The fisheries service has asked local boats to report unusual catches and is considering short-term restrictions on trawling in the worst-hit zones. Angelina Smith, who advises a regional fishing association, said an early closure would cost crews income but could prevent a longer shutdown if oxygen levels keep falling. Aerating the gulf is not considered practical: the affected water volume is too large, and the low-oxygen layer is spread across the seabed.
Forecast puts October in focus
The team’s worst-case projection puts the low-oxygen area at 1,200 square kilometres by 10 October if warm conditions persist and there is no strong wind-driven mixing. That would cover almost half the gulf. Marc Mazibuko, an ocean chemist who reviewed the measurements, said the estimate was a warning scenario, not a certainty, but stressed that the current trend was worse than seasonal records from 2019 and 2024.
A strong wind could mix oxygen into the lower water, researchers say, though it would not necessarily reverse damage already done to shellfish beds. If the oxygen deficit reaches the gulf’s main mussel-growing grounds, Smith said, growers could face a costly harvest interruption. The gulf supports fishing and shellfish businesses whose product is sold across northern Greece.
Researchers will repeat the station survey on Sunday and send an autonomous profiling instrument across the deepest part of the gulf. Suuronen-Tauriainen said the next readings should show whether the low-oxygen boundary is spreading or holding. “We need two more days of measurements before anyone can call this a temporary pocket,” she said.
