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Shift in Arabian Sea Plankton May Threaten Fisheries


Whether Noctiluca or overfishing is to blame, one major factor stands out: massive sewage flows into the Arabian Sea as the coastal population has exploded. As the study authors point out, Mumbai’s population has doubled to 21 million in the last decade. The region now sends 63 tons of nitrogen and 11 tons of phosphorus into the Arabian Sea each day. Karachi’s 15 million people send 70 percent of their wastewater into the sea untreated. Much of the fertilizer used to boost yields on farms in South Asia also eventually washes into rivers that drain into the sea.

“All of these cities are growing so rapidly, they don’t have the capacity to treat their sewage,” said study co-author Joaquim Goes, a biogeochemist at Lamont-Doherty. “The amount of material being discharged is humongous.”

Fish Catch in India
The Arabian Sea fishery may already be in decline. In Goa, India, women sort through the morning catch. © Joaquim Goes / LDEO, Columbia University

From the Gulf of Mexico to Chesapeake Bay, dead zones and degraded fisheries are on the rise globally. Doubling in size each decade, and now covering more than 95,000 square miles, they are “probably a key stressor on marine ecosystems,” according to a 2008 study in Science. Shifting ocean currents due to climate change can make the problem worse by dredging up nutrients from the ocean bottom.

In the Arabian Sea, stronger summer monsoon winds have boosted algae growth by bringing more nutrients from the deep ocean to the surface. In a 2005 study in Science, Goes, Gomes and colleagues showed that biomass from summer blooms off Somalia, Yemen and Oman, jumped nearly 350 percent between 1997 and 2004. They hypothesize that receding snow cover in the Himalaya-Tibetan plateau is making the Indian subcontinent hotter in summer compared to the Arabian Sea, strengthening the winds that blow toward India, bringing up more nutrients off Somalia, Yemen and Oman.

The researchers expected gentler monsoon winds in winter, as the process reversed itself, leading to fewer algae blooms. But NASA satellite maps showed just the opposite: more winter blooms. After several years of sampling what they thought were sporadic Noctiluca blooms, the researchers realized in 2006 that the blooms seen from space were not diatoms but recurring Noctiluca blooms.

They wondered if falling oxygen levels could explain the diatom-to-Noctiluca shift. Sure enough, the experiments aboard the Sagar Sampada seemed to confirm their hypothesis.

The study has attributed much of Noctiluca’s rise to growing sewage flows into the Arabian Sea, an intriguing connection that should be followed up on, says Andrew Juhl, a microbiologist at Lamont-Doherty who was not involved in the study. “It’s unusual for Noctiluca to bloom in the open sea and return year after year,” he said “All of these observations suggest that something dramatic has changed in the Arabian Sea.”

The study was funded by the National Science Foundation (NSF), NASA, Indian Space Research Organization (ISRO) and India’s Council of Scientific and Industrial Research (CSIR).

Other authors include Prabhu Matondkar, National Institute of Oceanography in Goa; Edward Buskey, University of Texas at Austin; Subhajit Basu, Goa University; Sushma Parab, Kent State University and Prasad Thoppil, Stennis Space Center.


Click here to read/download the Full Study – “Massive Outbreaks of Noctiluca scintillans Blooms in the Arabian Sea due to Spread of Hypoxia”.


Source: Columbia University.