Leopold Center for Sustainable Agriculture

Keeney lecture explores Gulf hypoxia issues

Back to Leopold Letter Winter 2009

By ALLISON SEVERSON, Communications assistant

One of the scientists who has measured the size and researched the science of the oxygen-deprived "dead zone" in the Gulf of Mexico every year since 1985 shared his findings at Iowa State University in November. Eugene Turner, Distinguished Research Master for the Coastal Ecology Institute and oceanographer at Louisiana State University, presented the 2009 Keenery Distinguished Lecture series, which honors Leopold Center founding director Dennis Keeney.

Turner spoke to an attentive audience, explaining everything from why starfish are changing their habits to why Iowa’s water quality correlates with Louisiana’s fisheries. He also put forth one idea that might improve water quality at least as much as other federal efforts.

“I propose a competition, a project involving all USDA employees to see which watershed can reduce pollution the most over 10 years,” he said.

Turner is a member of the Louisiana Universities Marine Consortium that takes measurements to determine the size of the “dead zone” every summer during a week-long cruise throughout the Gulf. The area was just over 3,000 square miles in 2009. It forms when excess nutrients, or eutrophication, promote algal growth. As dead algae decompose, oxygen is consumed in the process, resulting in low levels of oxygen in the water.

Hypoxic waters have dissolved oxygen concentrations of less than 2-3 parts per million. Turner noted that most ocean creatures try to stay out of low-oxygen areas, but some get trapped. Complicating factors are slope and ocean current.

“Starfish normally bury themselves in sediment on the ocean floor, but there is no oxygen there, and now they are pushing their way up above the surface,” Turner explained.

He reported hypoxia’s effect is most severe on diatoms, single-cell algae that are the most common types of phytoplankton and the basis of the aquatic food chain.

Turner pointed out that the nitrogen load in the Mississippi River as it enters the Gulf is directly proportional to the level of hypoxia in the Gulf of Mexico. “Nitrogen measurements in May will tell you what the hypoxia zone will be like in July. As of yet, climate is not directly playing a role,” he said.

The Gulf of Mexico has one of the world’s largest hypoxic zones and includes about 30 percent of U.S. fisheries. However, problems from excess nutrients are not confined to coastal areas and estuaries. In one study he cited, 31.8 percent of all streams in the United States are stressed by nitrogen; phosphorus was close behind at about 30 percent.

“A watershed problem requires a watershed solution. It will require a lot of people at the table, and we want that,” he said. “Louisiana will benefit or hurt directly from Iowa’s actions.”

What contributes to hypoxic zones?

Here is Turner’s list of factors and their role in creating conditions that lead to hypoxia:

Important factors:

  • Stratification
  • Currents
  • Winds, waves
  • Nutrient-enhanced primary production
  • High-flux of surface carbon to the seabed
  • Oxygen consumption exceeds oxygen resupply
  • Directly proportional to N load sediment carbon legacy

Unimportant (or minimal) factors:

  • Deep-water oxygen minimum layer
  • Allochthonous river carbon
  • Groundwater
  • Wetland erosion
  • Estuarine nutrients
  • Mississippi river deltaic levees
  • Reduced suspended sediments
  • Upwelled nutrients
  • Climate (not as yet)

Back to Leopold Letter Winter 2009