Predicting Dead Zones

by Bill Chameides | July 31st, 2009
posted by Erica Rowell (Editor)

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This year’s Chesapeake Bay dead zone was expected to be smaller than that of recent years, while the Gulf of Mexico’s was expected to be near record-size. Which area’s forecast was right?

Neither, but the more interesting question is why.

Spring Runoff Is a Key Determinant of Dead Zone Size

Predictions for dead zones (oxygen-depleted areas in the sea where fish and other marine life cannot survive) typically arrive in early summer after the all-important spring runoff data are in but before the zones reach peak size in mid-summer.

Spring runoff is a key determinant of size because the volume of runoff each spring roughly determines the amount of nutrients that will be delivered to nearby coastal areas where the dead zones form.

When spring flows are high, more pollutants from fertilizers and effluent from things like waste-water treatment plants pour into coastal areas. These additional nutrients feed large phytoplankton blooms, which in turn lead to large oxygen-depleted zones as the waste from these algal blooms is broken down by microbial action.

The opposite is generally true as well. Less spring runoff, less nutrients, smaller dead zones.

Gulf of Mexico Dead Zone

In the case of the gulf’s dead zone, high flows from the Mississippi River basin along with high nutrient levels this spring led experts to predict a near-record size dead zone this season. But, much to the surprise of those prognosticators, the dead zone’s area in July was found to cover about half the area it had covered for the last several years — about 3,000 square miles compared to 7,500-8,500 square miles. In fact, the areal extent of this particular dead zone for 2009 is among the smallest ever measured since 1985 when annual monitoring of the dead zone in the gulf began. Good news; the gulf dead zone is shrinking, right? Maybe not.

This year’s pattern appears to have been caused by “persistent winds out of the west that pushed low oxygen waters up against the Louisiana coastal shelf.” According to gulf hypoxia expert Nancy Rabelias, from Louisiana State University, the footprint of the Gulf of Mexico’s dead zone looks smaller, but the volume of ”dead” (oxygen-starved) water is still great. In other words, it’s not necessarily a smaller dead zone but one that’s more compact horizontally but deeper vertically.

Dead zone extent in the Gulf of Mexico 1985–2009.

Chesapeake Bay Dead Zone

In contrast to the gulf dead zone, the Chesapeake Bay dead zone is turning out to be larger than what was forecast. And this predicament appears to have occurred because the projections were based on data from the wrong rivers.

In most years, about 50 percent of the freshwater and much of the nutrients that flow into the Chesapeake comes from a single source: the Susquehanna River.

The Susquehanna’s two main sources, in upstate New York and western Pennsylvania, flow southward through Pennsylvania, entering the very northern extreme of the Chesapeake Bay from the northeastern corner of Maryland. As a result, the flow from the Susquehanna River is usually a pretty good indicator of what the dead zone in the bay will look like later that summer. And so the dead zone forecast for the Chesapeake Bay is largely based on this one river’s flow.

An Unusual Year With an Abnormally Wet Spring

But this has been an unusual year in many respects, including the amount of freshwater flow into the Chesapeake Bay, which, for most of the spring, was significantly below normal. As a result, the experts predicted this summer would see a smaller dead zone estimated to encompass a total of about 180 billion cubic feet* — the smallest size since 2001. (Source [pdf])

But what was not accounted for was the fact that the abnormally wet spring across Virginia and Maryland created enough runoff from rivers entering the bay further south to more than make up for the lack of flow from the Susquehanna. As a result, measurements from a June and early July cruise found a dead zone of about 282 billion cubic feet, a full 60 percent bigger than what had been predicted. This, as it turns out, is about the same as last year’s dead zone in the Chesapeake, which was bigger than the previous two years. (Source [pdf]) The final size of this year’s dead zone has yet to be published.

Go figure. It’s been a topsy-turvy season where unusual circumstances led to inaccurate forecasts.

Clearly scientists need to continue work to refine their forecasts. Generally scientists learn from their mistakes, and the anomalous weather encountered this year should lead to better forecasts in the future. The forecasts are important because they will help define how much nutrient runoff needs to be cut to shrink the dead zones to an acceptable size.

Regardless, the data from this summer show no signs that the dead zone problem is going to go away of its own accord.

End Note

* While the size of the dead zone in the gulf is typically reported as an area (i.e., square miles), the size in the Chesapeake Bay is reported as volume (i.e, cubic kilometers).

Further Reading

2009 Forecast of the Summer Hypoxic Zone Size, Northern Gulf of Mexico [pdf] (June 16, 2009)

Strange Connections: Midwest Farms and Floods Killing Gulf Fish? – (July 21, 2008)

An Update on the Gulf Dead Zone 2008 – (August 7, 2008)

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