The latest study on the health of the Chesapeake Bay has some encouraging news — offering signs that years of pollution-fighting efforts are having a positive effect. Now, it remains to be seen whether Congress is paying attention and can refrain from pulling the proverbial rug out from under the bay's cleanup campaign.

First the good news. A new study released by Johns Hopkins University and the University of Maryland Center for Environmental Science has found that efforts by Maryland and other states to reduce the flow of fertilizer, animal waste and other pollutants into the Chesapeake Bay has had a positive effect on the oxygen-deprived "dead zones" of the bay.

The largest such dead zone — near the Chesapeake's deep water channel — appears to have peaked in the 1980s and declined ever since, according to the study published this month. The researchers reviewed water quality records for six decades to form their conclusion — examining the nature of dead zones not only in the peak of summer when they are at their worst but in a longer time frame as well.

What appears to be happening, the authors conclude, is that efforts to reduce the flow of nitrogen and phosphorus are chiefly responsible for the beneficial effect. When fewer nutrients are dumped into the water (whether from sewage plants or septic systems, run-off from storms or the other myriad sources) there are fewer algae blooms to die and drain the oxygen.

The areas of hypoxia are not only getting smaller in size, but their duration is getting shorter as well. That's a remarkable achievement given that the six-state watershed has seen a rise in population and development during that time.

Even more encouraging is that the Obama administration's U.S. Environmental Protection Agency is willing to use existing authority under the Clean Water Act to press states for even more improvements. The so-called "pollution diet" proposed for the watershed states offers the most promising anti-pollution campaign in a generation and perhaps ever.

Yet that campaign is in peril, and Congress is its greatest threat. Last week, it came under attack in a hearing held by a subcommittee of the House Agriculture Committee by representatives who claim it would be too costly and that the science behind it is faulty.

That's a familiar political strategy — criticizing scientific research as insufficient — but particularly galling given the decades of study that have gone into the EPA's Chesapeake Bay model. The program could be seriously undermined if the committee yanks a conservation incentive offered to bay area farmers (first written into law in the 2008 Farm Bill) that helps pay for certain conservation practices that have provided a huge benefit to water quality.

Nor is this the only line of attack against the program. Other Republicans in the House would like to rewrite the Clean Water Act to reduce the EPA's regulatory authority. A lawsuit filed by the American Farm Bureau could upend the effort as well.

Make no mistake, it wouldn't take all that much to halt the EPA program in its tracks and reverse whatever modest improvements have taken place. There are any number of influential groups fighting hard against tougher pollution standards — from counties that would have to invest in more storm water controls to poultry companies that don't want to be held accountable for the manure produced by their chickens. All would love to see the program delayed, weakened or killed entirely.

But such criticisms seldom consider the benefits of a healthier Chesapeake Bay, not just for the creatures that inhabit the water but for the human beings that live near it. A cleaner environment is not just some "job-killing" ideal but represents potentially billions of dollars in value and thousands of jobs in tourism, including the fishing, crabbing and boating industries, real estate values and many other aspects of the local economy.

Marylanders stand to benefit as much as anyone from this. Without the federal government's involvement as regulator and promoter, states will be reluctant to make the kind of reforms that are needed to improve water quality further. For that to happen when the potential for a healthier Chesapeake Bay has been so clearly documented would be a tragedy beyond words.