Urban waste threatens fisheries in Arabian Sea

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  • Published 10.09.14
A close-up of the phytoplankton bloom in the Arabian Sea

New Delhi, Sept. 9: City wastes dumped into the sea by India and Pakistan may be driving massive blooms of tiny marine organisms called phytoplankton in the northern Arabian Sea that could threaten catch from one of India’s major fisheries zones.

A team of scientists from the National Institute of Oceanography (NIO), Goa, and collaborating institutions in the US warned today that these organisms have an extraordinary ability to thrive in oxygen-deficient seawater and may pose a threat to fisheries in the region.

The researchers used satellites and ships to observe the phytoplankton called Noctiluca scintillans that have emerged on vast swathes of the sea west of Gujarat and northern Maharashtra and south of Pakistan during the winter months year after year over the past decade.

The advent of this phytoplankton, which gives the surface of the sea an emerald green hue, appears linked to a decrease in the oxygen in the upper layers of the Arabian Sea, the scientists said in a paper published today in the journal Nature Communications.

“This phytoplankton thrives and proliferates in low oxygen conditions,” said Subhajit Basu, a research scholar and team member at Goa University who’s now preparing for a post-doctoral fellowship in marine sciences at Hebrew University in Jerusalem, Israel.

While phytoplankton lie at the base of the marine food chain, Noctiluca scintillans is a unique species that does not help boost fish population. It is fed upon primarily by jellyfish and salp, a tiny marine invertebrate. Neither are major sources of food for fish.

The phytoplankton itself and fecal pellets from the jellyfish and salp that eat the phytoplankton add more organic matter into the sea and further contribute to the loss of oxygen. “This is bad for other fish and may hurt fisheries,” said Prabhu Matondkar, an emeritus scientist at the NIO, who had first observed an unusual abundance of this phytoplankton in a satellite picture in the late-1990s.

“These blooms are massive, appear year after year and could be devastating to the Arabian Sea ecosystem over the long-run,” Helga do Rosario Gomes, a biogeochemist at Columbia University in New York who led the study said in a media release issued by the university.

Scientists at the Central Marine Fisheries Research Institute, Cochin, say the northern Arabian Sea is a key fisheries zone for India, having contributed over 850,000 tonnes — mainly catfish, ribbon fish, Bombay duck, cuttlefish, squid, octopus and threadfin breams — to India’s marine catch during 2012.

Fisheries sector analysts estimate that the northern Arabian Sea region accounts for about one-fourth of India’s marine fish catch.

While phytoplankton populations typically help support the marine food chain, scientists say things appeared to change in the northern Arabian Sea about a decade ago when vast blooms of Noctiluca scintillans appeared to coincide with drops in oxygen content of seawater.

The NIO-US researchers say no physical mechanisms can explain the appearance of oxygen deficient waters above 40-metres depth in the northern Arabian Sea during winter. One possibility, they speculate, is the domestic and industrial wastes dumped into the sea by India and Pakistan and other countries along the Arabian Sea rim.

Several studies have indicated that wastewater treatment plants across India have not kept pace with population growth. The scientists point out that Mumbai discharges over 2,700 million litres wastewater each day in the eastern Arabian Sea. Karachi releases about 1,600 million litres wastewater each day of which about 70 per cent is untreated, severely depleted organic-rich material.

Marine scientists are concerned that the emergence of Noctiluca scintillans will spawn an alternate food chain that lacks the predator fish that are staples of the fisheries industry.

“Something dramatic seems to have happened in the Arabian Sea,” said Andrew Juhl, a research professor of biology and environment studies at the Lamont-Doherty Earth Observatory at Columbia University, who was not associated with the study.

“It’s not unusual for this phytoplankton to bloom sporadically, but they’re typically smaller and do not occur in such a regular fashion — what’s being observed in the Arabian Sea suggests something in the ecosystem has changed,” Juhl told The Telegraph over the telephone.

“The suggestion that sewage may be responsible is an intriguing hypothesis — it needs to be followed up with research,” he said.

The NIO-US researchers spent three successive winters from 2009 through 2011 aboard the Sagar Sampada, an Indian research ship, sailing into the Arabian Sea, picking up samples of the phytoplankton and studying them in the ship’s laboratory.