The Gulf of Oman turns green twice a year, when an algae bloom the size of Mexico spreads across the Arabian Sea all the way to India.
Scientists who study the algae say the microscopic organisms are thriving in new conditions brought about by climate change, and displacing the zooplankton that underpin the local food chain, threatening the entire marine ecosystem.
Khalid al-Hashmi, a marine biologist at the Sultan Qaboos University in Oman, wrinkles his nose as the research vessel nears the bloom. "Sea stench," he says, referring to the algae's ammonia secretions. "It's here, you can smell it."
He signals the boat to stop as it speeds beneath a gigantic rock arch off the coast of Muscat, the capital of Oman, an arid sultanate on the southeastern corner of the Arabian Peninsula. The captain kills the engine and drops anchor into a slick of bright green muck surrounded by crystal-clear blue water.
The swarms of microscopic creatures beneath the surface of the Gulf of Oman were all but invisible 30 years ago - now they form giant, murky shapes that can be seen from satellites.
Across the planet, blooms have wrecked local ecosystems. Algae can paralyze fish, clog their gills, and absorb enough oxygen to suffocate them. Whales, turtles, dolphins and manatees have died, poisoned by algal toxins, in the Atlantic and Pacific. These toxins have infiltrated whole marine food chains and have, in rare cases, killed people, according to the UN science agency.
In the Great Lakes of North America, Thailand and the Seychelles, the algae bloom green. In Florida they are red, in the North Atlantic they are chalky white, and in Puget Sound they are orange. The Irish call it the "sea ghost," and the Taiwanese refer to the blooms as "blue tears."
NASA uses satellites and floating robots to monitor the blooms, said Paula Bontempi, the manager for ocean carbon and biology projects at the U.S. Space agency. "It's like a Van Gogh painting," she said, referring to satellite images of swirls of chlorophyll spiraling across the world's oceans. "Absolutely beautiful."
It's less attractive up close, where it can be "almost guacamole-like" in some lakes. "It smells bad, it looks bad," she said.
The satellite technology has enabled scientists to link the algae to higher levels of air and water pollution in recent decades, but Bontempi said questions remain. "We know that our Earth is changing," she said. "It may be in a direction we might not like.
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