When the Dead Zone Grows, It Really Grows; and When It Shrinks, It Shrinks

New research examines feedback effects and tipping points connected with low oxygen in the Bay

For the Chesapeake Bay, like mountains prone to avalanches, small disturbances can lead to big shake-ups. Scientists call such snowballing effects “positive feedbacks,” or when a move in one direction builds on itself and becomes amplified. A positive feedback can have negative consequences, and new research says that such feedbacks involving excess nutrients may be counteracting human efforts to clean up the Bay.

Feedbacks are a common feature in the Bay. Take, for example, Bay grasses. Feedbacks acting on these underwater plants, such as eelgrass or wild celery, can spell bad news for the Chesapeake. Those plants are good for the Bay, in part because they trap the dirt and silt that floats along the water column. But when the water quality in the estuary gets bad enough, those same plants will die. As a result, the Bay will get even dirtier and more plants will die — it’s a downward spiral.

Now, recent research at the University of Maryland suggests that another kind of feedback, involving chemical and biological processes near the Bay’s bottom, could be worsening the region’s infamous “dead zones.” These are regions of poorly oxygenated (or “hypoxic”) water that are deadly to animals and plants. Feedbacks involving nutrients like nitrogen or phosphorus could help to explain one of the Chesapeake’s biggest mysteries surrounding these zones: Nearly 30 years ago, and for reasons that scientists are still struggling to explain, the Bay became suddenly more susceptible to pollution from excess nutrients like nitrogen and phosphorus. That susceptibility, in turn, fueled the rise of more resilient and longer-lasting dead zones. Feedbacks, scientists say, could help elucidate how such a “tipping point” came about.

Dead zones emerge each year in coastal regions across the globe as nutrients wash off the land. Those nutrients kick off the growth of big algae blooms in estuaries like the Bay. And when those blooms die, they’re decomposed by bacteria, consuming large amounts of oxygen in the process. Although scientists have known for decades that dead zones existed, how feedbacks play a role in their expansion is a relatively new field, says Daniel Conley, a biogeochemist at Lund University in Sweden.

Conley studies dead zones both in the Chesapeake and in his native Europe. He saw evidence for the effect of feedbacks there, along the Danish Straits just east of Denmark, in 2002. That year, unseasonably warm weather gave rise to the largest dead zone ever measured in the country. Now, “there seems to be much more hypoxia than there was before, even though nutrient concentrations are the same,” he said. Feedback effects can help to explain that, he says.

Years ago and across the Atlantic Ocean from Denmark, such a shift also occurred in the Chesapeake. Scientists estimate that around 1986, nutrients began to take a bigger toll on the Bay. To create the same size dead zone, it now took only half the nutrients as before, according to a study published in 2004 by scientists at the University of Maryland Center for Environmental Science (UMCES). That’s likely why, even though humans have cut the nutrient pollution flowing into the estuary, the size of the dead zone has remained largely unchanged in recent years.  (One 2011 study does indicate that the dead zone has begun to shrink in the late summer months, slowly but surely. For more information on those trends, see an article in the December 2012 issue of Chesapeake Quarterly.)

The 1986 shift, and its suddenness, left researchers searching for an explanation. Michael Kemp, an ecologist at the UMCES Horn Point Laboratory, and one of his graduate students, Jeremy Testa, turned to the Bay’s basic chemistry and biology for an answer.

The scientists knew that as algae grow and die in the late spring and early summer, much of the nitrogen and phosphorus they carry sinks to the Bay’s floor. But that waste doesn’t just sit there. Under the right conditions, certain types of bacteria, called denitrifiers, consume many of those nitrogen molecules, expelling them as a harmless gas. Phosphorus, on the other hand, binds to the sediment at the Bay floor — and there it stays. In both cases, most of those nutrients are no longer a threat to the Bay.

But that’s during a good oxygen year, or a year when the dead zone remains relatively small. Under severe low oxygen conditions, however, experimental studies dating back to the 1990s have suggested that those same processes may not work as well. In other words, denitrifiers will digest and expel less nitrogen, while greater amounts of phosphorus will trickle up from the mud and into the water column. That, theoretically, could free up a lot more nutrients for algae to binge on, potentially fueling a second cycle of algae blooms. A dead zone becomes the perfect avalanche — one smaller zone of low oxygen might grow into a bigger one, largely thanks to feedbacks.

Or, as Testa puts it, “In a given year, [feedback] could maintain hypoxia” longer into the summer. If that’s the case, then a little nitrogen could go a long way in the Bay — a possible explanation for why, around 1986, fewer nutrients were needed to create the same-sized dead zone here.

Until recently, however, no one had shown whether such feedbacks were operating on a Baywide scale — or whether nutrients are recycled near the estuary’s bottom during oxygen-poor years, opening up more nutrients for algae to consume. So Testa and Kemp set out to do just that. The duo pored through data taken on the Bay from 1965 to 2007. They compared how much nitrogen was dumping into the Bay versus how much of the nutrient could be found at the bottom of the estuary in each of those years. 

And, sure enough, with less oxygen came more nutrients, the team reported in 2012 in the journal Limnology and Oceanography. During good oxygen years, nitrogen seemed to be removed from the ecosystem more easily — or, at least, Testa found a lot less of it near the Bay’s bottom. In oxygen-poor years, however, the opposite was true. Then, much greater percentages of nitrogen stuck around in the estuary. It was, he says, pretty good evidence that feedbacks did occur in the Bay.

Questions remain about the effect of such feedbacks on the Bay, Testa notes. It’s still not clear, for instance, how algae — which live at the surface — reach the nutrients which tend to sit near the Bay’s bottom. As a result, he’s still not sure how much of a role feedbacks played in the Chesapeake’s 1986 tipping point. Changes in climate and sea level rise likely had an impact, too. When it comes to the various factors driving the dead zone’s resilience. “We haven’t united them to try to understand how … one may be more important than the other in truly driving this increase in hypoxia,” he says.

Conley, for one, believes that feedbacks have been a major force in the growth and resilience of dead zones around world, including in the Danish Straits. The implications for those trying to clean up these water bodies could be big, he notes — anyone who hopes to shrink those dead zones will have to consider the additional trouble caused by positive feedbacks. And, in fact, scientists on the Bay have been doing just that. Researchers with the Chesapeake Bay Program, a federal and state partnership charged with overseeing the estuary’s clean up, have already incorporated positive feedbacks into their simulations of the Bay’s dynamics. Those simulations, in turn, have helped to determine how that same clean up will proceed.

“We’ve made estimates of how much nutrients have to be reduced in order to get the systems back on track,” Conley says. “And it might mean that we’d need to reduce them even further than we originally believed.”

Still, there’s some good news hidden in that dreary outlook, says Kemp. When the health of the Bay begins to drop, “the positive feedback tends to reinforce that trajectory,” he says. But when you start to improve water quality, “the same positive feedback mechanism tends to accelerate that recovery.” In other words, good changes can snowball, too.

If humans can heal the Bay enough, Kemp says, they may just push its positive feedbacks toward a positive, desirable direction. Think of it as an avalanche in reverse.

— Daniel Strain

Learn more: See Maryland Sea Grant's page on Ecosystems and Restoration.