Mucilage and Its Causes in the Sea of Marmara
Mucilage, also known as sea snot, is the sticky layer that builds up when stressed phytoplankton give off organic matter.
Mucilage, also known as sea snot, is the sticky layer that builds up on the surface and in the water column when stressed phytoplankton give off organic matter. In the Sea of Marmara the main causes are nitrogen and phosphorus from wastewater, warming water and a sea left still by weak winds.[3] It is not unique to the Marmara, and the first record comes from the Adriatic in 1729. The Marmara's worst outbreak came in 2021, and despite an action plan the mucilage returned in October 2024 and September 2025.[2][14]
How It Forms
Phytoplankton are the microscopic algae at the base of the marine food chain, and they normally give off small amounts of sugary, sticky material. Extra nitrogen and phosphorus push cell numbers up. In warm, calm weather the water does not mix, stressed cells produce more of the secretion, and it accumulates faster than bacteria can break it down. The material sticks together into large masses.[2][3] These show first at the surface and then sink, so mucilage that has left the surface has not left the sea.
The First Records in the Adriatic
The northern Adriatic has known this for longest. Records compiled from newspapers, magazines and scientific papers begin in 1729 and go on through 1872, 1880, 1891 and 1903. In recent decades it has come more often, with episodes in 1988-1991, 1997, 2000-2004 and 2006-2008. Roberto Danovaro and colleagues, publishing in 2009, looked at 200 years of Mediterranean records, found outbreaks climbing very quickly in the period 1990-2009 and tied this closely to temperature anomalies.[2][3] A large event was also reported at Thessaloniki in Greece in 2017.[2]
The Structure of the Marmara
Geography leaves the Marmara exposed. It has two layers, brackish water from the Black Sea on top and salty Mediterranean water from the Dardanelles beneath. The sharp boundary between them sits at 10 to 30 metres and limits vertical mixing.[2] The upper layer renews within months and the lower one takes years.[19] Oxygen in the lower layers is already low, as organic matter uses it up while decomposing on the way down. One review says oxygen-free water starts in places below 22 metres, and that migrating fish which passed easily at 40 metres twenty years ago can now meet it at 20 to 25 metres.[2]
The load comes from the shore. From the 1970s the Marmara region took in large-scale migration from the countryside, and cities and industry grew fast. Wastewater reaches the sea from the northern Marmara basin, the Gulfs of Izmit and Gemlik, the Nilüfer, Simav-Susurluk and Biga-Gönen catchments.[2] The region's population had reached about 27.2 million by late 2024.[12] Environment ministry figures cited in one report put wastewater flowing into the Marmara at about 5.7 million cubic metres a day, nearly half of it untreated.[11] The Marmara is also the only route for tankers to and from the Black Sea, and their pollution adds to the load.[3]
The 1997 and 2007 Events
Mucilage was first reported in the Marmara and the Dardanelles in 1997. It came back in mid autumn 2007 in the north-east of the sea, and over the winter of 2007-2008 it spread as white gelatinous material on the surface and through the water column, especially in the Gulf of Izmit.[2][3] These episodes usually faded within a month to 45 days.[4] So they were not treated as much of a warning, as the authors of a later review also observed.[2]
The 2021 Outbreak
Sources differ on when the largest outbreak began. A study of the meteorological record dates the first signs to December 2020, while Anadolu Agency and other reports say January 2021.[3][4] Barely visible over the winter, it grew in spring. Satellites picked it up first in the Dardanelles in early and mid March and then followed it towards the Gulfs of Izmit and Gemlik.[3] Pollution was not the only factor. Between March and June sea temperatures ran above normal at most stations, wind speeds fell, and the calm allowed the masses to collect along the shores.[3]
Halim Aytekin Ergül and colleagues took samples off Izmit and Biga in April 2021. Compared with September 2020, phytoplankton cells had risen from 2,000 to 50,000 per litre off Biga and from 12,000 to 42,000 off Izmit, while the number of species found in the Marmara fell from 59 to 28. The most abundant species at 10 metres off Izmit was the diatom Skeletonema costatum. The researchers reported that nitrogen was the limiting nutrient for phytoplankton that month.[1]
From mid May to the first week of June the cover grew quickly, reaching 12,742 hectares, about 127 square kilometres, on satellite measurements. By June the coasts were covered.[3][7] It was going down as well. A July 2021 study traced the masses below the salinity boundary as deep as 60 metres.[5]
Fishers and Marine Life
Those working at sea felt it first. In October 2021 Mustafa Altuğ Atalay, the agriculture ministry's director general for fisheries, set out the results before the Turkish parliament's mucilage commission. About 30% of mussels had died, corals had suffered serious harm, and fish eggs and larvae had fallen sharply. In a survey of 266 fishers in seven provinces the largest single loss, at 53.8%, was to nets, and fuel costs had risen by about 40%.[10] According to Prof. Mustafa Sarı, about half of the 200 or so purse seiners working the Marmara left for the Aegean and half for the Black Sea.[10]
On human health, a study of a limited number of samples found no toxic algae or toxicity to human cell lines.[6] Coastal towns were concerned that waterborne disease could spread if the outbreak went on.[3]
Effects on Ships
Ships around Istanbul, in the Marmara and in the Turkish Straits began to feel the effects in May 2021. One of the few studies on the subject lists seawater inlet chest filters, low-temperature coolers, oil coolers, ballast water filters, evaporators and emergency fire pumps as the first equipment affected. In a simulator test, a filter 60% blocked forced the main engine to slow down, while at lower levels the pumps raised the flow to hold the cooling.[8]
Whether ships help cause mucilage is a separate question. A 2021 preliminary study by Neslihan Balkıs-Özdelice and colleagues found species new to the Marmara in the mucilage and suggested they may have arrived in ballast water or with brackish inflow after heavy rain.[9] That is a possibility and not an established cause. The Ballast Water Management Convention has been in force since 2017, and compliance comes up in port state control inspections.
The Action Plan
Environment Minister Murat Kurum announced the 22-point Marmara Sea Protection Action Plan on 6 June 2021. At the time, 53% of the region's wastewater had only preliminary treatment, 42% had advanced biological treatment and 5% had biological treatment. The aim was to move all plants to advanced biological treatment within three years. Kurum also cited the view that cutting nitrogen in the sea by 40% could solve the problem. The plan further called for monitoring points to rise from 91 to 150, for rules on ship wastewater within three months and for ghost nets to be cleared within a year.[18]
Surface cleaning began on 8 June, and by 20 June about 5,000 cubic metres had been collected at 293 locations.[7] On 5 July Kurum reported 10,500 cubic metres from 592 locations, 8,570 inspections, 18 million lira in fines on 140 facilities and the suspension of 41 businesses.[13] At the coordination board meeting of 22 October he gave a total of 11,000 cubic metres, said no mucilage had been seen in early September, and set a target of raising treatment in the Marmara basin from 46% to 100% by 2024.[15]
The Outbreak Returns
When the plan's term ran out, its targets had not been met. Mucilage reappeared in October 2024. In December that year Prof. Melek İşinibilir Okyar reported a layer about 10 metres thick at about 22 metres off Marmaraereğlisi and said the spread resembled 2021. Prof. Ergül found masses at 10 to 15 metres in the Gulf of Izmit and blamed continuing household sewage discharges.[12] On 7 January 2025 Kurum put the responsibility on coastal municipalities, saying the plan's most important items had not been carried out and only 0.7% of existing plants had been converted to advanced treatment.[16]
The mucilage left the surface in June 2025. According to Prof. Sarı it was back three months later, on 14 September, with dense masses at 10 to 30 metres in the Gulf of Gemlik, off Tekirdağ and Marmara Island and in the Bandırma and Erdek gulfs. Sarı argued that the 2021 plan had failed and a new emergency plan was needed.[14] NASA's Landsat had already imaged a wide algal bloom on the Marmara on 5 September 2025.
The most detailed measurements came in 2025 from Deniz Kâşifi, an uncrewed underwater glider used by the Middle East Technical University's Institute of Marine Sciences. It stayed in the water for about 30 days, covered 600 kilometres, made 807 dives and collected data in the Çınarcık, Tekirdağ and Marmara Ereğlisi basins. It found that mucilage was produced continuously for a month, stayed at depth when it could not be seen on the surface, and had spread across much of the sea. The institute's director, Prof. Barış Salihoğlu, described this as a structural problem that needs permanent measures.[17]
Frequently asked questions
What is mucilage?
Mucilage, also known as sea snot, is the sticky layer formed when organic matter given off by phytoplankton builds up.
Why does mucilage form in the Sea of Marmara?
Nitrogen and phosphorus from wastewater feed phytoplankton, the two-layer structure limits mixing, and warm, windless weather helps the masses form.
Is mucilage toxic?
A study of a limited number of samples found no toxic algae or toxicity to human cells. The main harm has been to marine life through lost oxygen and to fishers through fouled nets.
Sources
- Ergül H. A. et al. The Early Stage of Mucilage Formation in the Marmara Sea During Spring 2021, Journal of the Black Sea/Mediterranean Environment, vol. 27, no. 2, 2021.
- Kayhan F. E., Yön Ertuğ N. D. Müsilaj Sorunu ve Karakterizasyonu, Doğa ve Sürdürülebilirlik Derneği, June 2022.
- Kömüşçü A. Ü., Aksoy M., Doğan O. H. An Analysis of Meteorological Conditions in Relation to Occurrence of the Mucilage Outbreaks in Sea of Marmara, International Journal of Environment and Geoinformatics, 2022.
- Anadolu Agency. 5 Things You Need to Know About Mucilage or Sea Snot, 2021.
- Istanbul University researchers. Vertical Distribution of Mucilage Typology in the Water Column After a Massive Mucilage Formation in the Surface Waters of the Sea of Marmara, 2021.
- Turkish Academy of Sciences. Ecology of the Marmara Sea: Formation and Interactions of Marine Mucilage, and Recommendations for Solutions.
- Global Voices. Marmara Sea Covered by Sea Snot, 23 June 2021.
- Usluer H. B. et al. Natural Threat to the Turkish Straits Sea Area: The Mucilage; Effect of Mucilage Pollution on Ship Cooling Systems: A Case Study.
- Balkıs-Özdelice N., Durmuş T., Balcı M. A Preliminary Study on the Intense Pelagic and Benthic Mucilage Phenomenon Observed in the Sea of Marmara, 2021 (abstract).
- BirGün. Müsilaj Nedeniyle Marmara Denizi'ndeki Midyelerin Yüzde 30'u Öldü, 21 October 2021; Balıkçılar Müsilajın Vurduğu Marmara Denizi'ni Terk Etti, 7 October 2021.
- Turkey Recap. Marmara Denizi'nde Alarm, 1 February 2024.
- Daily Sabah (DHA). Marmara Sea Faces Mucilage Problem Despite Efforts, 18 December 2024.
- bianet. Marmara Denizi'nin 592 Noktasında Müsilaj Temizliği, 5 July 2021.
- Bursa Hakimiyet. Marmara Denizi'nde Müsilaj Alarmı, 25 September 2025; 24 Saat. Marmara Denizi'nde Müsilaj 3 Ayda Yeniden Ortaya Çıktı, 24 September 2025.
- Sözcü. Bakan Kurum: 11 Bin Metreküp Müsilajı Toplayarak Bertaraf Ettik, 22 October 2021.
- Milliyet (DHA). Bakan Kurum Belediyelere Seslendi, 7 January 2025.
- Türkiye İş Bankası. Deniz Kâşifi Türk Denizlerinde İklimin Nabzını Ölçtü, 3 February 2026.
- Diken. Report on the Marmara Protection Action Plan, 6 June 2021.
- Beşiktepe Ş. et al. The Circulation and Hydrography of the Marmara Sea, Progress in Oceanography, 1994 (abstract); published studies of Marmara bays for renewal times.
