Lake Iznik, the fifth largest in Turkey, was recently closed to swimmers when its color changed to a strange green. The first findings of the phenomenon show that the color change is linked to the proliferation of cyanobacteria. The bacteria, also harmful to public health, spread over the lake this week while experts collected samples for further investigation of the issue.

Professor Meriç Albay from Istanbul University is among the experts. Albay said the lake was one of the most polluted in Turkey and various factors – excessive drainage of water, uncontrolled use of fertilizers and low water levels in streams supplying the lake – made cyanobacteria more visible.

The lake, located in the eponymous district of the northwestern province of Bursa, suffered from the seasonal drought last year, with water receding some 30 meters (98 feet) in places. Though rains replenished much of the water, it still faces a future risk of drought amid climate change concerns.

Professor Albay, who works at Istanbul University’s Faculty of Aquatic Sciences, said they have been conducting research on the lake since 1993 and it has been subjected to one of the "heaviest pollution" in Turkey. "It is surrounded by fertile agricultural lands but this means intense use of fertilizers which partially mixes with lake’s waters. In turn, they cause an increase in cyanobacteria. These organisms have species with toxin features," he told Demirören News Agency (DHA) on Tuesday. Albay highlighted that the lake should be better preserved and agricultural activities around it should be inspected carefully. "Measures are also needed for controlled water drainage, including groundwater," he added.