Istanbul, Turkey - The putrid smell descended on Istanbul's Kadikoy district one recent afternoon, seeping into restaurants and homes, wafting through parks and alleyways and lingering until the next morning.
For weeks, the dredging and industrial work taking place next to a heavily polluted creek entering the Marmara Sea on Istanbul's Asian shore sent millions of litres of polluted water into the sea, clouding once-pristine sailing waters. When temperatures rose, the smell of the dirtied water forced locals to close up their homes and steer clear of the seaside.
"It was something I have never experienced before in all my time living here," said Cenap Kuzuoglu, a restaurant owner in Moda, a section of Kadikoy popular with visitors. "We had this disturbing smell for the first three days after the heavy rain. It was more like sewage."
Kadikoy municipal authorities later confirmed the cause of the odour was sewage leaks, but the ongoing work would help stop mosquitoes breeding in the area. The city's water authority, meanwhile, has fielded separate complaints of foul-smelling tap water from Istanbul residents.
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Istanbul is surrounded by water on all sides, including the Marmara Sea to the south, home to important fishing grounds and sailing competitions. The Bosphorus Strait splits the city into its European and Asian districts, and is the passage through which dozens of fish species annually migrate north to the Black Sea to spawn.
A clean and healthy water system is crucial to Istanbul's tourism industry. The city was the world's sixth most-visited last year, with visitors contributing $8.6bn to the local economy. Tourists come to spot dolphins and view ancient ruins on boat trips along the Bosphorus, while the image of dozens of fishermen lining the Galata Bridge is one of Istanbul's most defining scenes.
But pollution of Istanbul's waterways could pose a serious threat to this historic city's lifeblood. With the population exploding in recent decades - from 1.5 million in 1960 to around 14 million today - future visitors may see more sewage and litter along the city's waterways, experts say.
In terms of marine litter, it's 250 percent worse now than 10 years ago.
"In terms of marine litter, it's 250 percent worse now than 10 years ago," Bayram Ozturk of the Turkish Marine Research Foundation told Al Jazeera, noting that the growth of ship traffic is largely to blame. Some 60,000 vessels traverse the Bosphorus Strait each year, discharging rubbish and fuel.
Unseasonable storms in June, meanwhile, forced authorities to release thousands of litres of sewage into Istanbul's waterways as drains and pipes overflowed. When a treatment plant next to the Kadikoy promenade, a coastal walkway frequented by locals, pumped its treated sewage into the sea to cope with the increased volume, the fetid smell overwhelmed residents.
According to the Turkish Statistical Institute, a government agency, 90 percent of the 1.1 billion cubic metres of treated wastewater discharged by Istanbul authorities in 2012, was released into the waters around the city. Berkay Peduk, an environmental engineer at ISKI, the city's water and sewage authority, said the volume of wastewater being treated daily in Istanbul has increased 16-fold since 1993, to 3.2 million cubic metres today. About 96,000 cubic metres is untreated.
"In spite of the fact that it looks like an enormous leap, we still don't call it enough," Peduk told Al Jazeera. The outdated treatment plant in Kadikoy, for instance, only removes waste particles larger than 10 millimetres, leaving smaller but equally damaging material. But Peduk maintains that sea pollution is not a result of the discharge from treatment plants:
"Biological treatment plants are so advanced that they … eliminate big particles and decrease the nitrogen, phosphorus and carbon levels in the wastewater into environmentally friendly conditions."
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A staggering 300,000 people move to Istanbul from elsewhere in Turkey every year - double the entire national growth rate - and the average resident discharges around 224 litres of wastewater each day, compared to 435 litres in New York City and 160 litres in London.
Many fish species were lost over the years due to the discharge of wastewater.
Human waste is rich in nitrogen and phosphorus - elements that spur bacteria growth, which in turn uses up oxygen that fish need to survive. Polluted water also stops natural light from reaching underwater plants.
"Many fish species were lost over the years due to the discharge of wastewater," Turgut Onay, an environmental science professor at Bogazici University in Istanbul, told Al Jazeera. "It takes a long time to restore the sea's water quality."
Thirty-eight percent of Turkey's industry is located in Istanbul, where pollution has resulted in algal growth and jellyfish blooms, the latter blamed for blocking pipes and damaging fishing nets. When the upper reaches of Istanbul's Golden Horn waterway served as a centre for industrial factories and manufacturing plants in the 1980s and early 90s, industrial sludge was openly released into the sea.
Since then, city authorities have spent $653m to clean up the 10km-long Golden Horn, and in 2004, the city embarked on a series of projects to clean up Istanbul's sewage systems. In 2012, a new $147m water treatment plant opened in the west of the city, bringing the total number of plants to 67. Eighty swimming areas in and around Istanbul are being tested this year by the city's water authority, which says water quality levels are broadly acceptable.
Meanwhile, with water levels at reservoirs reportedly reaching their lowest in decades, Turkey's undersecretary for environment and urban planning, Mustafa Ozturk, has called on residents to use water sparingly this summer. His comments, however, have been contradicted by the minister for forestry and waterworks, who said in August that drought issues were not a major concern.
Higher water temperatures brought on by climate change have also allowed jellyfish populations to explode, reducing the availability of oxygen and food for other species.
The recent odour invasion occurred amid temperatures of 31C, and local newspapers have warned of a months-long stench resulting from the riverside construction in Kadikoy, a district of around 1.5 million people. While local restaurant owners say pedestrian traffic has not yet declined, others believe long-term threats could emerge to the tourism industry - and to Istanbul's aquatic ecosystem.
While fishermen still catch fish on the iconic Galata Bridge, they are down to a single species, Aksit Ozkural of the Turkish Marine Environment Protection Association told Al Jazeera. "And we don't know if the dolphins that once stayed here are doing that any more," Ozkural added. "Because we see fewer dolphins, we presume that's not the case."