Seagrass, for all its benefits, is not the most glamorous of ocean life. The underwater wisps, in varied shades of green, grow in shallow, sunlit water along coastlines. In time, they can mature into dense meadows, supporting a wide array of life—baby fish, crabs, worms, anemones, and all that eat them, including birds. Mostly, though, seagrasses are overlooked and forgotten, out of sight and mind, even as they decline by 7 percent globally year after year. In the Chesapeake Bay, however, seagrasses have scientists sitting up and taking notice. That’s because, for the first time in decades, submerged aquatic vegetation (SAV, in scientific parlance) is returning. A century’s worth of fertilizers washed into the bay killed most of the grass by the 1980s, and for decades scientists have tried to reduce pollution, with little success. But now, after the U.S. Environmental Protect Agency (EPA) spearheaded a new effort in 2010, seagrass is back. A recent paper, published in...