Plankton 'Dead Zone' May Threaten Arabian Fisheries

Sea plankton are taking over waters in the Arabian Sea, creating a "dead zone" that is threatening the region's fisheries, according to new research.

For the last decade, Noctiluca scintillans, a unique type of plankton that is suited to low-oxygen waters, have colored the Arabian Sea an emerald green each winter, from the shores of Oman on the west, to India and Pakistan on the east. These thick blooms create an oxygen-starved dead zone the size of Texas that is potentially dangerous to predator fish in the area, which sustain 120 million people living on the sea's coast.

"These blooms are massive, appear year after year, and could be devastating to the Arabian Sea ecosystem over the long-term," study lead author Helga do Rosario Gomes, a biogeochemist at Lamont-Doherty, said in a statement.

Until recently, photosynthetic diatoms, a type of algae, supported the Arabian Sea food chain. But vast blooms of Noctiluca, which first appeared in the early 2000s, brought with it a drop in the number of diatoms, effectively reducing the amount of oxygen in local waters and causing a major shift in the Arabian ecological food chain. Based on experiments, the researchers found that Noctiluca's carbon-fixation rates rose by up to 300 percent while the diatoms' fell by nearly as much when the two inhabited the same waters.

And these formed dead zones are not just isolated to the Arabian Sea. From the Gulf of Mexico to Chesapeake Bay, dead zones and declining fisheries are on the rise globally, doubling in size each decade, and now covering more than 95,000 square miles.

Researchers suspect that a tide of nutrient-rich sewage flowing from booming cities on the Arabian Sea is expanding the dead zone and feeding Noctiluca's growth.

"All of these cities are growing so rapidly they don't have the capacity to treat their sewage," said study co-author Joaquim Goes. "The amount of material being discharged is humongous."

The findings were published in the journal Nature Communications.