The Chesapeake Bay's dead zone may have finally begun to heal, but progress could depend on the weather.

FEW PEOPLE WILL EVER SEE A CRAB JUBILEE IN MARYLAND. During these events, whose comings and goings are hard to predict, blue crabs, by the dozens or hundreds, scuttle out from the deep and up the banks of the Chesapeake Bay. They sit there in the shallows, right on top of each other and often for hours. For those who are lucky enough to be walking by and like seafood, they're easy pickings.

But, scientists say, such jubilees are no party. In fact, they're a sign that something's rotten down below. Namely, the crabs are escaping the Bay's dead zone, a wide region of water lying along the estuary's deeper channels that each year becomes stripped of most of its oxygen — or, as scientists would say, the water turns hypoxic. This happens because excess nutrients such as nitrogen and phosphorus spill off the land and into the estuary, kicking off biological and chemical processes that form the dead zone. Winds will sometimes push those bottom waters up and into the Bay's shallower regions, forcing crabs to get moving in search of water with enough oxygen for them to survive. And so the crab jubilee begins.

But one recent report has some good news for those crabs and for people, too. The size of the Bay dead zone has shown signs of shrinking, at least in the late summer, researchers say. It may not be the end of crab parties, but it's a start. At the same time, scientists are also struggling to understand why that zone hasn't shrunk more, especially as humans have cut the nutrients they've sent down to the estuary since the mid-1980s.

The changes seen in the late summer dead zone represent "a slow decline," says Michael Kemp, an ecologist at the University of Maryland Center for Environmental Science's Horn Point Lab. "But it's enough that we can statistically measure the changes."

From his office in Gloucester Point, Virginia, Robert Diaz can see a lot of dead zones — but few that are shrinking. Diaz, a marine scientist at the Virginia Institute of Marine Science, is part of a project that tracks the formation of oxygen-poor, or hypoxic, zones in the Chesapeake Bay and worldwide. Generally, water is considered hypoxic if it carries less than two milligrams of dissolved oxygen per liter (most fish need about three milligrams per liter just to survive). So far, he and his colleagues have listed around 500 sites that fit that criterion. The team gives each one their own dot on a map in Google Ocean.

The Chesapeake Bay's own dot was first reported in 1938. Then, scientists taking early dissolved oxygen measurements recorded the first hints of low oxygen in some of the Bay's waters. The phenomenon was later called an "oxygen desert." This desert, eventually renamed a "dead zone," had been expanding across the Chesapeake for decades. But, beginning in the 1980s, its growth tapered off. In recent years, "the Bay has just about kept even with the population growth and the other pressures on it," Diaz says. While that's a victory of sorts, he notes, the estuary should have made bigger gains.

Humans, after all, have reduced the excess nutrients flowing into the Bay — and, as a result, the size of the dead zone should have shrunk. The reasoning is this: Every year, nutrients like the nitrogen in farm fertilizers and factory gas emissions dribble down to the Bay during the rainy season. There, they kick off a chain reaction. Algae consume the nutrients come spring, and, when those algae die in the summer, bacteria binge on their remains, consuming huge quantities of dissolved oxygen in the process. Voilà, you've got your dead zone. So if you reduce the nutrients, you should reduce its size. In recent years, those sorts of cuts have come from a number of different sources, including sewage treatment plants, farms, and factories (see What Goes Up Must Come Down).

The red zone is the dead zone, where the water is anoxic, empty of oxygen. The orange and yellow zones are hypoxic regions, holding a little more oxygen but still not enough for a healthy, life-supporting ecosystem. When the wind shifts, low-oxygen water can slosh out of the deeps, creep up into the shallows, and send blue crabs scuttling up onto shore, creating a spectacle known as a "crab jubilee" (see the photograph at the top of this article). Photograph of crab jubilee in Delaware by Kevin Fleming; map courtesy of EcoCheck, data provided by the Chesapeake Bay Program.

But the dead zone didn't go away. In fact, things actually got worse. Over the decades, scientists have noticed, something has been altered about the way that hypoxia builds up in the Bay. The end result is that in recent years, the same amount of nitrogen has given rise to twice the volume of dead zone than in the past. Scientists have pinpointed that tipping point to around 1986. According to Diaz, who's seen similar trends in other dead zones around the world, it's possible that these ecosystems have been fundamentally changed by decades of pollution. "These large systems have been hypoxic now for such a long time that something has changed about them... [so] that it takes less new nitrogen coming in nowadays to create the same-size dead zone," he says.

But new research suggests that the Bay might not be so broken after all.

That comes from a research team at Johns Hopkins University who worked with Kemp to test a new idea: could the dead zone be growing or shrinking during only one part of the summer but not another? In other words, was the dead zone holding as steady in June as it was in July? The team was onto something, it turns out.

The researchers reported in 2011 that today's dead zone seems to be as big and nasty, on average, as it was in the mid-to-late 1980s — and maybe a bit bigger. But that bad news story described only the early summer months. In the late summer, or from mid-July on, the news was better. The dead zone seemed to have begun to mellow. To be precise, the late summer dead zone measured, on average, about nine cubic kilometers in the 1980s. By the 2000s, that number had shrunk down to about seven cubic kilometers, the researchers reported in the Estuaries and Coasts. In even more good news, the dead zone also seemed to be sticking around for less time, too, lasting for 110 days, on average, in the summer as opposed to 130.

The bottom line is that the Bay might not be as stubborn to change as some scientists thought. To be sure, the reduction in size of the late-summer dead zone was relatively small. Even so, "if we hadn't controlled the nutrients, things would be even worse," says William Ball, an environmental engineer at Johns Hopkins whose graduate student, Rebecca Murphy, spearheaded the study.

But some big questions remain: despite these hopeful signs, why has the dead zone been relatively resistant to change? What happened in 1986 so that fewer nutrients could create the same-sized dead zone, and why was the dead zone shrinking in late July but not June?

Malcolm Scully thinks the answer to these dilemmas largely comes down to those windy days out on the Chesapeake. "[For] anyone who's looked at a lot of data or even probably spent time in a boat out there, the wind is hugely important," says Scully, a physical oceanographer at Old Dominion University.

Important because winds are like a silver bullet to the heart of the dead zone. A good, strong wind can mix up the Bay, pushing surface waters around and drawing bottom waters up. Those hypoxic waters can then refill their depleted oxygen levels by absorbing the gas from the atmosphere. So while some winds may be bad for crabs — sending them scrambling on a jubilee — they're beneficial for the Bay as a whole.

Scully got a good look at that phenomenon in 2011. He left an array of sensors for monitoring levels of dissolved oxygen out on the Chesapeake during Tropical Storm Irene. While the storm swept over the Bay in August of that year, the scientist watched as the entire dead zone, which had been sizable, disappeared. Oxygen levels leapt up, beginning along Maryland's Western Shore and moving east.

Not all winds are created equal when it comes to their effect on the Bay, Scully says. Southerly winds, or those that blow up from Norfolk toward Annapolis, tend to act a lot more like Irene did. They mix up the Bay. A lot. But around 1980, southerly winds became less common on the Bay, while westerly winds became more so, and they didn't stir the water quite as much. That comes down to how wind direction, in combination with the rotation of the Earth, sloshes water around inside the Bay. Luckily, southerly winds seem to have increased again in recent years, reaching the normal levels seen before 1980. Those changes were driven by a shift in atmospheric pressure around Bermuda, Scully reported in a 2010 paper in the Journal of Physical Oceanography.

Such climatic shifts would likely have the biggest effect on the Bay in the early summer, too. During that season, the estuary is usually more stratified than at other times — its bottom waters, which tend to be cold and salty, stay separate from the waters above, which tend to be warmer and fresher. It's somewhat like how oil and water don't mix in a jar. For reasons that remain unclear, too, that June stratification has grown even stronger over the past several decades, slowing the natural mixing of the Bay's waters at that time. Winds blowing from the south would help to mix those waters up, but they came less often beginning in 1980. That means that the dead zone would likely have been bigger, or more stubborn, in the early summer than scientists expected — just what the Johns Hopkins team and Kemp had found.

And the decrease in southerly winds in 1980 would help to explain why, several years later, it began taking fewer nutrients to build up the same-sized dead zone. During the early summer, "it would appear that things are not getting better even though the nitrogen loads have come down," Scully says. But "if you account for winds, as well, you explain a lot of that."

The science isn't settled, however, and there are other explanations for the dead zone's resistance to recovery. Rising sea levels could explain why the Bay's waters have become even more stratified, especially in June. Higher sea levels send a bigger flux of salt water into the estuary, helping to keep the salty bottom and fresher surface waters more separate than otherwise.

And, as Diaz suggested, there's a chance that the biology and chemistry of the Bay have been altered, too. Specifically, the consistently low oxygen levels around the estuary may have made it more difficult for bacteria, plus chemical processes, to remove nitrogen and phosphorus from the water column and the sediments below. That, in turn, leaves more nutrients free for algae to consume, leading, ultimately, to less oxygen in the water. In other words, when excess nutrients are added to the Bay, they may make the estuary even more susceptible to nutrient pollution — something scientists call a "positive feedback." Such a feedback could help explain the sudden shift seen around 1986. "This positive feedback couldn't have caused this doubling of hypoxia," on its own, says Jeremy Testa, a graduate student who studies, among other things, nutrient recycling in the Bay with Michael Kemp. "But it could certainly support it once the wheels are set in motion."

Diaz, for his part, buys all of these explanations. "I think it's a mix of all of these things," he says. But regardless of how stubborn the dead zone is, restoration is possible, Diaz adds. "I don't think the dead zone will ever go away. But I do think it can be reduced in size."

Now, that may be a reason to party. Don't forget to invite the crabs.