The oil that befouled the Gulf of Mexico for 86 days is vanishing from the surface... But scientists warn the oil's ecological impacts are shifting, not ebbing, thanks to massive volumes of dispersants... That dispersed oil now hovers, diluted in the water column, posing a challenge for scientists to track and measure the subsea plumes. Mapping the long-term effects of the nearly 2 million gallons of dispersant used by BP PLC may well be equally difficult, given the array of unanswered questions that surround the products' rapid breakdown of oil droplets and their chronic toxicity... A May meeting at the University of New Hampshire's Coastal Response Research Center, planned by government scientists and oil industry representatives, yielded a consensus judgment that dispersant use "has generally been less environmentally harmful than allowing the oil to migrate on the surface into the sensitive wetlands." Another group of scientists, however, issued a public plea last week that decried dispersants and warned that, mixed with oil, the products "pose grave health risks to marine life and human health."