Karachi’s Foul Smell Linked to Decaying Phytoplankton, Experts Say

KARACHI, PAKISTAN — WEB DESK: A strong and unusual odour that spread across large parts of Karachi has been linked to the natural decomposition of microscopic marine organisms along the coast, with weak winds allowing the smell to travel and linger far beyond the shoreline, according to marine and weather experts.

Residents woke on Wednesday to a pungent, often fish-like smell across several parts of Pakistan’s largest city.

The odour was reported from coastal neighbourhoods including Clifton and DHA, but was also noticeable farther inland in Korangi, PECHS, MA Jinnah Road, Tower and Kharadar.

The widespread smell prompted questions over whether Karachi was experiencing an unusual pollution incident, gas leak or another environmental problem.

Marine experts, however, pointed to a seasonal phenomenon in the Arabian Sea.

What Is Causing Karachi’s Foul Smell?

WWF-Pakistan Technical Adviser on Marine Resources Muhammad Moazzam Khan identified decomposing phytoplankton as the principal source of the smell.

Phytoplankton are microscopic marine organisms that occur naturally in the sea and form an important part of aquatic ecosystems.

Their abundance can increase during the monsoon. When marine conditions change as the season ends, large numbers can die and begin decomposing, producing a strong and unpleasant odour.

Khan said changes in wind, water conditions and sea temperature can affect the marine vegetation system and contribute to the die-off.

Other marine vegetation that flourishes during the monsoon can also decay and add to the smell.

Why Did the Smell Reach So Far Into Karachi?

The marine decomposition alone does not fully explain why residents far from Karachi’s coastline could smell it.

Weather conditions played an important role.

According to the Pakistan Meteorological Department, sea breezes were unusually weak from Tuesday night into Wednesday morning.

Karachi normally receives southwesterly winds from the Arabian Sea that help move and disperse coastal air.

With weaker winds, the odour was able to remain over the city instead of dispersing quickly.

Climate expert Fatima Yamin also pointed to a low-pressure area off India’s Jamnagar coast that was affecting wind circulation around Karachi. She said air pollution and prevailing weather conditions were contributing to the circulation of the smell.

Sea Breeze Expected to Help Clear Odour

Conditions began changing later on Wednesday.

Southwesterly sea breezes strengthened during the evening, reaching speeds of up to 22 kilometres per hour, according to the Tribune’s report citing meteorological information.

The PMD expected the sea breeze to become more active from Thursday.

Stronger coastal winds should help disperse the lingering odour and improve conditions across affected areas.

WWF-P’s Khan told Dawn that the smell would likely dissipate within a few days as marine and atmospheric conditions changed.

Is the Smell Dangerous?

Based on the expert explanations currently available, there is no indication that the odour represents a sudden toxic leak or mysterious industrial incident.

The phenomenon has been attributed primarily to naturally decomposing phytoplankton and other marine vegetation combined with weather conditions.

WWF-P experts have therefore said there is little reason for alarm over the marine phenomenon itself.

However, this should not be interpreted as a general statement about Karachi’s overall air quality. Separate pollution and smog conditions can still affect the city independently of the phytoplankton-related smell.

Has This Happened in Karachi Before?

Yes.

According to WWF-P’s Muhammad Moazzam Khan, Karachi experienced a similar phytoplankton-related phenomenon in 2024.

The occurrence is associated with seasonal changes in the marine environment as monsoon conditions retreat.

The latest phenomenon also appears not to be confined to Karachi.

Khan said a similar occurrence had been observed along the Gwadar coast approximately three days earlier, suggesting seasonal marine changes were affecting more than one section of Pakistan’s coastline.

What Are Phytoplankton?

Phytoplankton are microscopic organisms that live primarily in sunlit upper layers of oceans and other bodies of water.

Dawn, citing the WWF-P expert, reported that they occur in the upper 100 metres of the sea, where sufficient sunlight penetrates, and cannot generally be seen individually with the naked eye.

They play an essential role in marine food webs and are a natural part of the ocean ecosystem.

During favourable conditions, however, their populations can increase rapidly. When environmental conditions subsequently change, large quantities can die.

The decomposition of this organic material can generate the distinctive smell residents noticed across Karachi.

Why Did Seawater Appear Greenish?

Large concentrations of phytoplankton can sometimes alter the appearance of seawater.

The Express Tribune reported that during the monsoon, phytoplankton can multiply to concentrations capable of giving coastal water a greenish appearance.

That does not automatically indicate chemical contamination.

The current expert assessment points toward a seasonal biological process rather than evidence of a newly identified chemical spill.

Karachi’s Unwelcome Visitor Expected to Fade

The combination of a seasonal phytoplankton die-off and unusually weak sea breezes appears to explain why Wednesday’s smell became noticeable across such a large section of Karachi.

The most strongly supported explanation from current reporting is that phytoplankton and other marine vegetation proliferated during the monsoon, began decomposing as marine conditions changed, and weak winds prevented the resulting odour from dispersing normally.

With stronger sea breezes returning, experts expect the smell to gradually dissipate rather than persist as a long-term phenomenon.