Experts warned that the oxygen-depleted layer at the bottom of the Marmara Sea is gradually moving toward the surface and noted that failure to prevent the increasing pollution would be a disaster for the sea.

According to the data from Middle East Technical University’s (METU) Institute of Marine Sciences, the oxygen level in the first 30 meters (98 feet) of the Marmara Sea, where the waters from the Black Sea are present, is measured at 8-9 milligrams per liter, while it drops to 1-2 milligrams per liter in the Mediterranean waters starting from 30 meters, and dips below 1 milligram per liter in the layer from the seabed up to 300 meters below the surface.

Based on the long-term data compiled by a project dubbed the Marmara Sea Integrated Modeling System (MARMOD) conducted by METU, it was found that the oxygen levels in the Çınarcık basin in eastern Marmara have dropped to below 0.5 milligrams per liter.

In comparison, in the 1980s, it was noted that these levels were measured above 2 milligrams per liter.

Professor Mustafa Yücel, deputy director at METU’s Institute of Marine Sciences, explained that oxygen dissolves in smaller amounts at certain temperatures and salinity levels, and particularly, oxygen in the deep waters has decreased below the threshold levels in the last 30-35 years.

He emphasized that the decrease in oxygen levels to below 2 milligrams per liter means that most commercial fish species cannot survive in this water and will flee.

Not only fish but also zooplankton, which play a significant role in the food chain and respiration, require oxygen, and their habitats are shrinking.

Yücel pointed out that the habitats of marine organisms native to the Marmara or species using it as a migratory route have also significantly narrowed.

"The cause of oxygen scarcity is primarily pollution. Harmful algal blooms intertwine with pollution, leading to further oxygen scarcity. The ultimate outcome could be deteriorated organic layers, such as the infamous 'sea snot’ or marine mucilage, causing foul-smelling, discolored waters, where even your hand disappears underwater," Yücel said.

Yücel additionally pointed out that the consequences of the pollution also include damage to economic activities in the sea, such as fish farms, and the loss of many services obtained from the sea, affecting everything from tourism to aquaculture.

Climate change effect

Professor Barış Salihoğlu, the director of METU’s Institute of Marine Sciences, stated that the Marmara Sea is essential from a socio-economic perspective, transitioning between the physically and ecologically distinct waters of the Mediterranean and the Black Sea.

He noted that throughout history, the sea has experienced significant human pressure, which has been increasing, with pollution and overfishing being the most substantial factors.

"The pressure of climate change is also very intense; this is already a global pressure and it is felt very strongly in Marmara," he said.

Despite the negative effects of the Black Sea and taking the pollution coming from the Danube River under control, Salihoğlu said that in contrast to this, the pressure of the cities surrounding the sea has significantly increased.

"As a result, the Marmara Sea has turned into a ‘sick sea.’ Oxygen below 25 meters does not sustain marine life. At present, it is almost anoxic. This suggests that the sea conditions will be extremely harsh during the summer. Oxygen has depleted at 28 meters; there seems to be significant organic accumulation just below the oxygenated layer, which has consumed the oxygen through decomposition," he explained.

"Oxygen levels slightly increase in deeper areas, but they remain at hypoxic levels. If we reduce pollution by half today, we can aim that the water from the Mediterranean, which feeds the Marmara Sea, will raise the oxygen levels above the hypoxic threshold in at least five or six years," said Salihoğlu.

He stressed the urgency of upgrading all treatment systems to advanced treatment and cleaning the waters of the 11 rivers that flow into the Marmara Sea to avoid the worst-case scenario.

"The oceans provide us with two types of ‘services.’ One is directly translated into money – obtaining food from the sea. The other is the oxygen and health aspects, which do not have a direct place in the market but carry significant economic and social value. We are losing these services now. The oceans absorb less carbon dioxide and produce less oxygen. They may start producing disturbing toxic gases," he concluded.