Shift in sea plankton may threaten fisheries.

• Position: Arabian Sea

A growing "dead zone" in the middle of the Arabian Sea has allowed plankton uniquely suited to low-oxygen water to take over the base of the food chain. This rise to dominance over the last decade could be disastrous for the predator fish that sustain 120,000,000 people living on the sea's edge.

Scientists at Columbia University's Lamont-Doherty Earth Observatory, Palisades, N.Y., and their colleagues are the first to document the rapid rise of green Noctiluca scintilians, an unusual dinoflagellate that eats other plankton and draws energy from the sun via microscopic algae living within its cells. The thick blooms of Noctiluca color the Arabian Sea an emerald green each winter, from the shores of Oman on the west to India and Pakistan on the east.

The researchers show how the millions of green algae living within the cells of Noctiluca allow it to exploit an oxygen-starved dead zone the size of Texas. They hypothesize 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. "These blooms are massive, appear year after year, and could be devastating to the Arabian Sea ecosystem over the long term," says biogeochemist and lead author Helga Gomes.

Until recently, photosynthetic diatoms supported the Arabian Sea food chain. Zooplankton grazed on the diatoms, a type of algae, and in turn were eaten by fish. In the early 2000s, it all changed. The researchers began to see vast blooms of Noctiluca and a steep drop in diatoms and dissolved oxygen in the water column. Within a decade, Noctiluca virtually had replaced diatoms at the base of the food chain, marking the start of a...