The extreme flooding of the Mississippi River in spring 2011 might result in the largest-ever dead zone in the Gulf of Mexico, according to a June 14 report by a team of NOAA-supported scientists from the Louisiana Universities Marine Consortium, Louisiana State University and the University of Michigan.
EarthSky spoke with biological oceanographer Nancy Rabalais of the Louisiana Universities Marine Consortium. Dr. Rabalais led researchers in generating a 2011 forecast in the Gulf of Mexico for hypoxia – oxygen-starved waters more commonly known as a ‘dead zone,’ hostile to sea creastures. She said:
The forecast for this year is pretty simple. We’re predicting it’s going to be the biggest one ever, since we’ve started mapping the area in 1985, which means that it could be as large as 26,000 square kilometers, which would be about 9,400 square miles. The largest we’ve gotten to date has been 22,000 square kilometers, which is about 8,500 square miles.
A dead zone, or hypoxic zone, is an oxygen-starved area of water. It’s caused by excessive nitrogen runoff from farmland fertilizers and livestock waste washing into rivers and then into the ocean. Nitrogen fuels the rapid growth of large populations of algae and plankton. When they die and sink to the bottom, their decay robs the water of oxygen. That results in too little oxygen to support most marine life in bottom and near-bottom water. Dr. Rabelais said:
The area of low oxygen that’s often called the dead zone is a water mass off the Louisiana shore that extends from the Mississippi River, far to the west, well onto the Texas shore. It is an area where there is not enough oxygen in the bottom waters to support marine life that we would be familiar with, such as fish, shrimp, and crabs. It extends from very close to shore to about anywhere from 60 to 120 miles offshore, from shallow water fifteen feet deep out to about 120 feet deep.
Mapping of the Gulf’s dead zones began in 1985. The largest measured so far, in 2002, was more than 8,400 square miles.
During May 2011, stream-flow rates in the Mississippi and Atchafalaya Rivers were nearly twice that of normal conditions. This significantly increased the amount of nitrogen transported by the rivers into the Gulf. According to USGS estimates, the amount of nitrogen transported to the Gulf in May 2011 was 35 percent higher than average May nitrogen loads estimated in the last 32 years. Dr. Rabelais told EarthSky:
There’s definitely a connection between the flooding and what we expect to see off-shore this summer. There are several things that help the formation of the low oxygen, and one is the fresh water. That has certainly gone up. The other is the nutrient levels, which have gone up with the fresh water flow. There are several people who have made forecasts based on the Mississippi River nutrients, particularly nitrates, which is in the dissolved form and runs off of the land much more readily. And those predictions show a very close correlation between the amount of nitrogen that comes into the Gulf in May to the area of the low oxygen that are mapped in July. And these predictions are very strong. They explain over 80 percent of the variability and size from year-to-year.
With the discharge this year being extremely high and well over the maximum discharge since 1930, it probably rivals the 1927 flood as well. There are just more and more nutrients coming into the Gulf, which means more phytoplankton are going to grow. So more phytoplankton, more organic matter getting to the bottom, more consumption of oxygen by bacteria and more severe and intense and over broader areas for the low oxygen to occur.
Listen to the 8-minute and 90-second EarthSky interviews with Nancy Rabelais on why the 2011 Gulf of Mexico dead zone might be biggest ever (at top of page.)