Residents of Karachi, particularly those residing near the sea, woke up on Wednesday morning to a foul smell. The cause — decaying microscopic marine algae and current weather conditions.

Muhammad Moazzam Khan, technical adviser on marine resources at World Wide Fund for Nature-Pakistan (WWF-P), said the fishy smell was a result of decomposing microscopic marine algae called phytoplankton — a naturally occurring event.

He explained that when the regional monsoon system ends in mid-September, it results in significant changes in the sea conditions. Consequently, these phytoplankton, found abundantly during the monsoon, begin to decay.

Phytoplankton are found in the upper 100 metres of the sea, where sunlight can penetrate, and are not visible to the naked eye.

The WWF-P expert said this phenomenon — also witnessed in 2024 — often causes a foul smell to permeate the air in the coastal areas. He added that the smell would likely dissipate in a few days.

The Pakistan Meteorological Department’s (PMD) spokesperson, Anjum Nazir Zaigham, and climate change expert Fatima Yamin attributed the scale of the smell to the weather conditions too.

Yamin said a low-pressure area off the coast of India’s Jamnagar was circulating wind around Karachi’s coastline, which was turning into smog because of air pollution.

She added that the combination of smog and the weather conditions was causing the smell to “circulate throughout the city”.

Zaigham also pointed out the low-pressure area and the smog, in addition to the south-easterly wind direction.

He said the air would likely get better by the evening with the sea breeze.

The stench was also reported across the metropolis in October 2024, with Clifton, Defence Housing Authority (DHA), SMCHS, PECHS, Korangi, Orangi, North Nazimabad, Federal B Area (blocks 9 and 10), and Saddar witnessing it strongly.

Asked about possible health risks at the time, ecologist Rafiullah had said that while there was no “concrete” evidence yet, prolonged exposure to the combination of dusty winds and the smell might affect sinusitis.