Foul Smell Along Karachi Coast Linked to Decaying Phytoplankton: WWF-Pakistan

KARACHI, PAKISTAN — WEB DESK: An unpleasant smell detected along parts of Karachi’s coastline can be caused by decomposing phytoplankton, according to a marine expert associated with WWF-Pakistan, highlighting a recurring seasonal phenomenon along the port city’s Arabian Sea coast.

Muhammad Moazzam Khan, technical adviser at WWF-Pakistan, has previously explained that the distinctive odour occurs when large quantities of phytoplankton that have proliferated in coastal waters begin to die and decompose.

The phenomenon has been recorded repeatedly along Karachi’s coastline, particularly around seasonal transitions before and after the monsoon.

Phytoplankton are microscopic organisms that live in marine and freshwater environments and form a fundamental part of aquatic food webs. Through photosynthesis, they also play a major role in global oxygen production and carbon cycling.

Why Does Karachi’s Coast Sometimes Smell?

According to WWF-Pakistan’s previous explanations of similar events, large concentrations of phytoplankton develop in waters off Pakistan’s coast under favourable environmental conditions.

When the organisms die and wash towards the coast, their decomposition can produce a distinctive unpleasant smell. Changing winds can then carry that odour from the coastline into different parts of Karachi.

This means the smell does not necessarily indicate that dead fish, sewage or an industrial chemical release is responsible.

However, without testing a specific episode, other possible sources of odour should not automatically be ruled out.

What Is Phytoplankton?

Phytoplankton are microscopic organisms that drift in oceans, rivers and other bodies of water.

Despite their small size, they are crucial to marine ecosystems because they form the base of many aquatic food chains.

They use sunlight and carbon dioxide during photosynthesis and release oxygen. WWF describes phytoplankton as major primary producers that contribute substantially to the oxygen in Earth’s atmosphere.

One organism repeatedly associated with unusual coastal smells in Karachi is Noctiluca scintillans, commonly known as “sea sparkle.”

WWF-Pakistan has previously documented large blooms of this organism in the northern Arabian Sea. When such blooms collapse and the organisms decay, an unusual smell can spread along the coast.

A Recurring Karachi Phenomenon

The occurrence is not unprecedented.

In September 2024, residents around Hawksbay, Sandspit, Turtle Beach, Ibrahim Hyderi and Keamari reported a strong smell after monsoon rains and tidal activity. WWF-Pakistan attributed that episode to decomposing marine organisms, particularly phytoplankton.

Another episode occurred in October 2024, when an unpleasant odour was reported across parts of Karachi after changing weather and wind conditions.

WWF-Pakistan again linked the smell to decomposing phytoplankton and said wind direction helped transport the odour across different areas of the city.

Similar episodes were documented much earlier.

In 2018, WWF-Pakistan linked a strong smell across Karachi to the collapse of a phytoplankton bloom involving Noctiluca scintillans after changing currents and wind patterns.

In 2017, WWF-Pakistan similarly attributed an unusual coastal stench to a decaying sea-sparkle bloom following changes in currents and winds associated with the monsoon.

Seasonal Winds Can Spread the Odour

Wind direction is an important factor in determining where the smell is noticed.

Phytoplankton may decompose close to the coastline, but changing winds can carry the odour much farther inland.

WWF-Pakistan’s Khan previously explained that post-monsoon wind changes can transport the smell across Karachi before it gradually dissipates.

This helps explain why residents far from the immediate shoreline may sometimes report the same smell.

The intensity and duration can vary depending on the size of the bloom, decomposition, tides and prevailing winds.

Is the Smell Dangerous?

During an earlier Karachi phytoplankton episode, Khan said the smell itself was generally not harmful and could disappear within a day or two.

That historical assessment should not, however, be treated as a medical evaluation of every current exposure.

People experiencing persistent breathing difficulties, severe irritation or other symptoms should rely on appropriate health guidance rather than assuming their symptoms are caused solely by phytoplankton.

Certain intense phytoplankton blooms can also have ecological consequences. WWF-Pakistan has previously noted that some blooms may be associated with fish or shellfish mortality, although this does not occur in every event.

Why the Timing Matters

Pakistan’s coastline is influenced by seasonal changes in the Arabian Sea, including monsoon-driven changes in winds and currents.

WWF-Pakistan has previously reported that phytoplankton blooms can occur during both pre-monsoon and post-monsoon periods, with changing currents contributing to the beaching and decay of marine organisms.

Karachi residents may therefore encounter the distinctive smell periodically, particularly when large blooms coincide with environmental conditions that push decomposing material towards the shore and carry the resulting odour inland.

The repeated historical observations support phytoplankton decomposition as a plausible and well-documented explanation for such coastal odours in Karachi. A definitive attribution of any specific current episode, however, is strongest when supported by direct sampling or a contemporaneous assessment by marine or environmental authorities.