NOT SO long ago, the Chesapeake Bay seemed unsaveable. Punished by years of population growth and polluted runoff, the bay’s waters were murky. Crucial species were dying off, and oxygen had become so scarce that toxic “dead zones” proliferated. Because water flowed from so many states into the largest watershed on the Eastern Seaboard, an effective response was hard to imagine.

Yet now, nearly a decade after the federal government stepped in with a not-particularly-onerous conservation effort, the bay is rebounding in a record-setting way. “We provide conclusive evidence that reducing discharges of nitrogen, phosphorus and other pollutants into the bay has produced the largest resurgence of underwater grasses ever recorded anywhere,” the authors of a new study write. “This success shows that coastal ecosystems are resilient and that concerted efforts to reduce nutrient pollution can result in substantial improvements.”

The study reported that bay grasses have rebounded fourfold since 1984, including in areas that have not hosted submerged vegetation in years. Underwater grasses provide a crucial habitat for a wide range of aquatic life, from seahorses to snails. Maryland’s famous blue crabs feed on the life that the grasses support.

The resurgence in underwater vegetation is just one benefit of the “pollution diet” the Environmental Protection Agency created for the bay’s watershed, which includes six states and the District. By also altering urban surfaces to prevent excessive runoff and upgrading wastewater treatment plants, the plan has helped cut nitrogen pollution 23 percent and phosphorus pollution 8 percent. “Nutrients overfertilize the bay, creating huge blooms of algae that die and deplete oxygen from the water,” the authors explained. These nutrients also contribute to murkiness that blocks the sunlight grasses need.

Part of what makes the Chesapeake Bay program distinct from more traditional pollution control efforts is its proposed idea of “nutrient trading,” a flexible approach to meeting the watershed’s pollution diet. This allows, say, a wastewater plant that would have to spend a great deal of money complying with pollution limits to pay farmers to plant cover crops that reduce their nutrients instead. In this way, the easiest pollution reductions come first and the expensive ones are given more time.

President Trump has repeatedly insisted that he wants “crystal clear water.” The Chesapeake Bay’s cleanup program is beginning to produce just that, and it promises to demonstrate the value of efficient, market-based pollution controls, too. Yet Mr. Trump zeroed out the program in his budget. Thankfully, Congress appears poised to maintain funding.

Notwithstanding its recent progress, the bay still suffers from significant oxygen-depleted dead zones, excessive nitrogen runoff and other problems. Farm pollution from fertilizer and manure continues to be a major challenge. A sustained federal commitment is essential.

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