\u00a0<\/strong>In the Arabian Sea, a growing \u201cdead zone\u201d linked to water pollution has allowed plankton uniquely suited to low-oxygen water to take over the base of the food chain. Their rise to dominance over the last decade could be disastrous for the predator fish that sustain 120 million people living along the sea\u2019s edge.<\/p>\n Lamont\u2019s Helga do Rosario Gomes<\/a>, Joaquim Goes<\/a>, and colleagues were the first to document the rapid rise of this plankton, called green\u00a0Noctiluca scintillans. <\/em>It is an unusual dinoflagellate that eats other plankton and draws energy from the Sun via microscopic algae living within its cells.<\/p>\n Winter blooms of Noctiluca are so vast they can be seen from space. (Norman Kuring, NASA)<\/p><\/div>\n In a 2015 study<\/a>, the scientists hypothesize that a tide of nutrient-rich sewage flowing from booming cities on the Arabian Sea is feeding\u00a0Noctiluca\u2019s\u00a0<\/em>growth by expanding an oxygen-starved dead zone the size of Texas. They show how the microscopic algae living within Noctiluca<\/em> cells allow the plankton to exploit this dead zone.<\/p>\n In winter, Noctiluca\u2019s\u00a0<\/em>thick blooms color the Arabian Sea an emerald green, from the shores of Oman to India and Pakistan. Until recently, photosynthetic diatoms supported the Arabian Sea food chain; zooplankton grazed on the diatoms and were, in turn, eaten by fish. In the early 2000s, that changed. The researchers began to see vast blooms of\u00a0Noctiluca\u00a0<\/em>and a steep drop in diatoms and dissolved oxygen in the water. Within a decade,\u00a0Noctiluca\u00a0<\/em>had virtually replaced diatoms at the base of the food chain, marking the start of a colossal ecosystem shift.<\/p>\n To understand the key to\u00a0Noctiluca\u00a0<\/em>\u2019s success, the researchers spent three successive winters sampling blooms and performing experiments. Putting Noctiluca\u00a0<\/em>and its diatom competitors in oxygen-starved water, they found that\u00a0Noctiluca\u2019s\u00a0<\/em>carbon-fixation rate rose by up to 300 percent while that of the diatoms fell by nearly as much.<\/p>\n Arabian Sea fisheries have suffered during many of the same years. Whether\u00a0Noctiluca\u00a0<\/em>or overfishing is to blame, one major factor stands out: massive sewage flows into the Arabian Sea as the coastal population has exploded. Mumbai\u2019s 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\u2019s 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.<\/p>\n From the Arabian Sea to the Gulf of Mexico to Chesapeake Bay, dead zones are on the rise globally and now cover more than 95,000 square miles. Shifting ocean currents due to climate change can make the problem worse by dredging up nutrients from the ocean bottom.<\/p>\n \u201cIt\u2019s unusual for\u00a0Noctiluca\u00a0<\/em>to bloom in the open sea and return year after year,\u201d said Andy Juhl<\/a>. \u201cAll of these observations suggest that something dramatic has changed in the Arabian Sea.\u201d<\/p>\n Ocean Acidification<\/strong><\/p>\n Rising carbon dioxide levels are another challenge affecting oceans and marine life worldwide.<\/p>\n The oceans have taken up about a quarter of the carbon dioxide humans have put in the atmosphere over the last two hundred years. As CO2 <\/sub>levels have increased, the acidity of ocean water has risen, with serious consequences for shell-building creatures such as oysters and tiny pteropods that are a key part of the marine food chain.<\/p>\n Studies show that as carbon dioxide levels have increased in the atmosphere over the past two centuries, seawater has become less saturated with aragonite and calcite, which several kinds of sea creatures use to build their skeletons and shells. The average pH of seawater has also fallen from about 8.2 to 8.1, about a 30 percent increase in acidity on pH\u2019s logarithmic scale.<\/p>\n![\"Winter](\"http:\/\/blog.ldeo.columbia.edu\/2015report\/files\/2014\/05\/arabian-sea-blooms_nasa-750x578-300x231.jpg\")