Earlier this year, scientists said the 2011 dead zone in the Gulf of Mexico might be the biggest ever. But the 2011 dead zone – the zone of oxygen-depleted water that forms every year in the Gulf of Mexico – has been measured by these same scientists to be smaller than predicted, according to a release issued August 4, 2011. It’s possible that Tropical Storm Don disrupted the Gulf dead zone when it passed through in late July.
Dead zones are areas of oxygen-depleted waters that form when nutrients stimulate the growth of algae blooms. Algae eventually die, sink and decompose and the decomposition process, driven by microbial respiration, consumes large amounts of oxygen. The lack of oxygen can cause large die-offs of fish and other invertebrates and it poses a threat to recreational and commercial fisheries.
The dead zone in the Gulf of Mexico is triggered by nutrient-rich discharges of nitrates and phosphates from the Mississippi River. Sources of nitrates and phosphates include fertilizer runoff from agricultural crops, animal waste and human sewage. In June, scientists issued a report suggesting that extreme flooding of the Mississippi River in spring 2011 might result in the largest-ever dead zone in the Gulf of Mexico. The report came from a NOAA-supported team of scientists from the Louisiana Universities Marine Consortium, Louisiana State University and the University of Michigan.
The size of the dead zone in the Gulf of Mexico has averaged 6,688 square miles over the past five years. In 2011, scientists predicted that the dead zone would expand to between 8,500 and 9,421 square miles due to heavy spring flooding in the Mississippi River drainage basin and increased nutrient runoff.
Scientists measured the size of the dead zone in late July and early August 2011 during a research cruise led by Nancy Rabalais, executive director of the Louisiana University Marine Consortium, and discovered that the dead zone was only 6,765 square miles – smaller than predicted based upon their forecasting model. They believe strong wind and waves associated with Tropical Storm Don mixed the Gulf with oxygen-rich waters and disrupted the development of the dead zone. The current forecasting model does not incorporate short-term variability due to strong weather events.
Robert Magnien, director of NOAA’s Center for Sponsored Coastal Ocean Research, commented in an August 4, 2011 press release:
Despite fluctuations in size due to each year’s weather conditions, these chronic, recurring hypoxic zones every summer represent a significant threat to Gulf ecosystems. Until we achieve a substantial reduction in nutrient pollution from the Mississippi River watershed, we will continue to experience extended periods of time each year when critically-needed habitat is unavailable for many marine organisms.
In mid-August, NOAA plans on sponsoring a follow-up cruise to collect an updated measure of the size of the dead zone in the Gulf of Mexico.
Deanna Conners is an Environmental Scientist who holds a Ph.D. in Toxicology and an M.S. in Environmental Studies. Her interest in toxicology stems from having grown up near the Love Canal Superfund Site in New York. Her current work is to provide high-quality scientific information to the public and decision-makers and to help build cross-disciplinary partnerships that help solve environmental problems. She writes about Earth science and nature conservation for EarthSky.