Several residents have complained of a foul odor in the metropolis since Tuesday evening.A foul stench permeated several parts of the city including Defence Housing Authority (DHA), Clifton, Shireen Jinnah Colony and parts of Keamari late on Tuesday evening and lingered on till Wednesday evening. “I went to Malir to see my ailing sister , but when I returned I smelt some very strange smell like rotten fish and it felt everywhere in the area,” said Muhammad Hanif, a shopkeeper in Shireen Jinnah Colony. Experts attributed this stench to the collapsing of phytoplankton bloom caused by Noctiluca. “This planktonic dinoflagellate is commonly found along Pakistan’s coast and is known to form large blooms, especially during the pre-monsoon (April-May) and post-monsoon (September) along Pakistan,” said Muhammad Moazzam Khan, Technical Advisor (Marine Fisheries), WWF-Pakistan. A large bloom of this phytoplankton was present in the northern Arabian Sea during September, as reported by observers posted by WWF-Pakistan on fishing vessels. The end of the monsoon season and changes in currents and wind patterns during the last few days have resulted in beaching and collapse of this bloom on the coast. This resulted in the spread of a decaying seaweed smell in the city.A similar foul smell spread in Karachi on 31 May 2017, because of a similar collapse of bloom of Noctilucascintillans. The mass bloom of this small free floating organism was reported from the Arabian Sea including the Pakistan coast. Noctilucascintillans blooms, commonly known as sea sparkle, have been reported from Pakistan’s coast on many occasions. They occur in two forms, i.e. orange (or red) and green, which sometimes result in the mortality of fish and shellfish. During the current year the mass scale green bloom occurred but no mortality of fish or shellfish has been reported from Pakistan so far. “The bloom of Noctilucascintillans occur at least twice a year,” Khan said, adding that in some years, the intensity of such bloom increases substantially and results in fish mortality in some cases. He also said that the onset of the monsoon results in a change in current pattern and due to this Noctiluca starts dying and results in an atypical odour. ‘The smell is not harmful generally,’ he added. The frequency of such blooms is increasing in Pakistan, which may be attributed to climate change. However, other anthropogenic factors may be the cause of such blooms. According to Khan the stench caused by this bloom will vanish in a day or two.Published in Daily Times, October 11th 2018.