Ocean ‘dead zones’ hard to predict

Nov. 6, 2009, 10:25PM



  Scientist stymied when trying to forecast size and severity

Most dead zones, including the large recurring one in the Gulf of Mexico off the Louisiana coast, are produced largely by fertilizer runoffs carried by rivers into the ocean.

The fertilizer stimulates an overgrowth of algae that sinks, decomposes, and consumes most of the life-giving oxygen supply in the water. At an average of 6,000 square miles, the Gulf of Mexico dead zone is among the country’s largest and threatens fisheries and recreation.

Yet although ocean scientists understand the basic drivers of these oxygen-starved waters, researchers’ ability to predict the size and severity of dead zones remains relatively poor.

In June, a group of scientists from the Louisiana Universities Marine Consortium, Louisiana State University, and the University of Michigan predicted the 2009 dead zone south of Louisiana would be the largest, or second-largest of the last 20 years.

Instead it was the fourth-smallest.

To address this, the National Oceanic and Atmospheric Administration recently gave five-year, multimillion-dollar grants to the Louisiana-led group that made the errant 2009 prediction as well as a group led by Steve DiMarco, an oceanographer at Texas A&M University, to improve their dead zone models.

“This should allow us to take a big step forward,” DiMarco said.

The Louisiana modeling team, led by Nancy Rabalais, made its predictions after observing large amounts of nitrogen feeding into the Gulf from the Mississippi and Atchafalaya rivers.

Typically, Rabalais said, the amount of fertilizer flowing into the Gulf accounts for 81 percent of the variability in year-to-year size of the dead zone.

But this year, abnormally persistent winds out of the west pushed the nutrient-rich waters of the Mississippi off the continental shelf — where dead zones form — into deeper waters where they do not.

Thus the weather, which is largely unpredictable more than seven days in advance, messed up the forecast by Rabalais’ group.

DiMarco, whose group has its own models to predict dead zone size, said this past summer’s smaller-than-forecast dead zone highlights the peril of forecasting them solely on nutrient levels flowing down rivers.

“There are multiple facets of what’s causing the dead zone,” he said. “We’re not just looking at the influence of nutrients coming down the Mississippi. Our team has physical oceanographers, trying to characterize the system in its entirety.

“We have tended not to put out a prediction because predicting the size of the dead zone is almost like predicting the weather. We don’t know how good our model will be a month in advance,” he said.

With the influx of funds, both groups will beef up their survey cruises — typically in June, July and August — to better characterize the size, depth and other critical features of the dead zone.

For example: How, precisely, does current speed impact dead zone formation? The idea, DiMarco said, is to sort out all of the key processes that lead to dead zone formation and dissipation in order to more accurately model their behavior.

The long-range goal is not so much to produce a summer dead zone forecasts, but rather to model the dead zone in its entirety.

With such a model scientists could better understand how well certain interventions, including lowering fertilizer usage, would actually affect the size of the dead zone.

That, DiMarco said, would give NOAA and other government agencies interested in addressing the country’s growing problems with dead zones the tools they need to devise effective countermeasures.