b'SURFACE WATERRising Carbon, Falling pHCarbon dioxide levels in the atmosphere haveBut even before the pH falls low enough to be increased by about one-third since the start of thecorrosive, acidified seawater reduces the rate at which Industrial Revolution, from about 300 parts per millioncarbonate precipitates out of solution. Precipitated (ppm) to about 400 ppm. About one-third of thecarbonate is the fundamental ingredient used by atmosphere\'s carbon dioxide dissolves into the ocean,shell-building organisms to make their says Tyburczy. As dissolved carbon dioxide levels haveprotective armor. increased, the earth\'s seawater has become about 30 percent more acidic.If the carbonate saturation state of the water is low because of excess CO 2 , fragile larval oysters and other For creatures that use carbonate from ocean water tobivalves and shellfish are forced to expend too much build protective shellsincluding shellfish like oystersenergy to build their shells. and snails, urchins, crabs, coccolithophores (plankton) and pteropods (tiny "sea butterflies" that are an"It takes more energy than they have," Tyburczy says. important food for wild salmon)acidic water poses"They don\'t have yolk sacsthey get their energy from two grave threats, Tyburczy notes. Most dramatically,feeding on plankton. If they can\'t make ends meet acidic water can corrode shells.with their energy budget, then they\'ll die or become so weakened that they can\'t fight off even low levels of parasites or bacteria." It\'s not hard to see why Tyburczy is so anxious to find out how underwater plants could combat acidification.Joe Tyburczy and Kiana Encinas set up a SonTek Argonaut-ADV on a PVC rig to measure the flow of water across a bed of eelgrass.Photo: Michael Sundman40 MISSION: WATER'