Goals Unlikely To Protect Gulf Of Mexico Shrimp Industry

Researchers from U-M, Louisiana State University, and Limnotech Inc, an Ann Arbor-based firm, used three different models to analyze oxygen depletion and to answer two key questions: is the expanded dead zone human-caused, and will the proposed 30 percent nitrogen load reduction be sufficient to reduce the zone to below 5,000 square kilometers, as agreed to by federal, state and tribal leaders in 2001.

The hypoxic region is an area where water lacks sufficient oxygen to sustain most marine life, and in the Gulf of Mexico it is caused by excess nitrogen•largely runoff from mid-west agriculture, said Donald Scavia, director of the Michigan Sea Grant College Program and professor in the School of Natural Resources and Environment.

Scavia’s paper, published in the June edition of the journal Estuaries, found that the 30 percent nitrogen load reduction will not likely shrink the dead zone to the desired 5,000 square kilometers. According to the paper, the nitrogen load must be reduced by 40 percent to 45 percent to achieve that reduction in most years.

Comparing the results of the three models also confirmed anecdotal and sparse historic data indicating that large-scale hypoxia did not occur before the mid-1970s, and supports the notion that tripling the nitrogen load over the past 50 years has led to the heightened Gulf of Mexico hypoxia problem.

Confidence in the model analysis was bolstered this year as the operational ecological forecast from the National Oceanic and Atmospheric Administration, based on Scavia’s model, predicted this summer’s dead zone to be 5,400 square miles. Measurements from the NOAA-supported surveys by the Louisiana Universities Marine Consortium documented the zone to be 5,800 square miles, or about the size of Connecticut.

Hypoxia occurs when increased nitrogen runoff causes algae blooms, which sink into bottom waters and are decomposed by bacteria, a process that consumes oxygen. The warm fresh water from the Mississippi and Atchafalaya rivers also layer atop the colder salty Gulf waters, preventing atmospheric oxygen from getting to the bottom. As oxygen is consumed faster than it can be supplied, concentrations decrease below the critical 2 mg/l that defines hypoxia and has resulted in collapses of fisheries in other parts of the world. It’s important to reduce the size of the dead zone in the Gulf because the area is important habitat for shrimp and other important fin and shellfish.

Hypoxia and other problems caused by excess nitrogen load are not unique to the Gulf of Mexico. Resent NOAA reports indicate that this problem occurs in more than 50 percent of US estuaries and the United Nations Environment Program has identified nitrogen overload and its contribution to the rapid growth of oxygen-starved zones in some coastal waters as an emerging global issue.

For more information on Michigan Sea Grant

Scavia's faculty profile: http://www.snre.umich.edu/faculty-staff-directory/faculty-detail-print.php?faculty_id=195