What is a Dead Zone?
A dead zone or hypoxic zone is an area of lake or ocean that does not have enough oxygen for most marine life to survive. The fish and other animals that are able to escape leave the area. Crustaceans and other bottom-dwelling critters that aren’t so mobile, suffocate and die.
Oxygen dissolved in water is typically expressed as a percentage. Water that is hypoxic generally has an oxygen concentration of 1-30% (most fish and marine life cannot live under 30%), anything below 1% saturation is known as anaerobic, reducing, or anoxic. Healthy marine environments seldom have less than 80% oxygen saturation.
What causes Dead zones?
Dead zones are caused by an influx of certain nutrients like nitrogen and phosphorus, common chemicals used for agricultural fertilizers. These nutrients runoff the soil into groundwater and rivers. These rivers carry the nitrogen and phosphorus to their mouths where they meet with big lakes or oceans.
The influx of these nutrients assists the overgrowth of algae. Too many algae in the water deplete oxygen by blocking light out of the bottom and through decomposition. The bacterias that are responsible for algae decomposition deprive the water of oxygen.
The Gulf of Mexico Dead Zone
The Gulf of Mexico is a 617,763 square mile gulf that opens to the North Atlantic ocean. The Gulf of Mexico has one of the world’s largest dead zones that fluctuates in size year after year. In 2018 it covered an area of 2,720 square miles, 2019 it measured 6,952 square miles and in 2020, it measured 2,116 square miles and was the 3rd smallest in the 34-year record of surveys. The mouth of the historic Mississippi River flows into the Gulf of Mexico, where it collects the majority of its pollution.
Throughout American history, the Mississippi River has been a passageway for goods from the port of New Orleans all the way up to Minneapolis and beyond. It stretches from the tip of the northern arm of Lake Itasca south through the midwest and out through Louisiana into the Gulf of Mexico.
Most grains produced in the US are grown in the midwest (The Corn Belt is a region of the Midwestern United States that has dominated corn production in the United States since the mid 19th century). These row crops are fertilized with chemical fertilizer made from nitrogen or phosphorus that runs off the soil into the Mississippi.
Waste runoff from large cities like New Orleans, Memphis, Baton Rouge, and St. Louis combined with excessive fertilizer runoff from crop fields in the midwest creates an abundance of essential nutrients (nitrogen and phosphorus in particular) for aquatic plants, distinctly algae.
Since 1986 the National Oceanic and Atmospheric Association or NOAA, has forecasted and studied the gulf’s dead zone. In 2017 they recorded the largest dead zone size since they started studying the area, 8,776 square miles.
How Dead Zones Effect The Fishing Economy
Large areas where fish and other sea stock is severly depleated impacts a lot more than just nature. Tens of millions of dollars of seafood is lost to these dead zones ever year. Fishing towns and fisheries are heavily effected when their supply of fresh fish and shellfish is disrupted.
Louisisna pulls upwards of a billions dollars of fish and shellfish from the gulf of mexico every year. Most of the wild shrimp caught and consumed in the US comes from the gulf of mexico. Theres a distrubing pattern where as the hypoxic zones in the Gulf fluxuate so does the annual fish landing. Shrimp landing have almost halfed in volume since the turn of the century accounting for a losses of over millions of dollars.
Large Dead Zones around the world
Hypoxic zones in our oceans have quadrupals since the 1950’s. The largest dead zone in the world stretches across the entire 63-thousand mile gulf of Oman in the Arabian Sea. The Baltic sea dead zone off the coast of Sweden stretches from Poland all the way to finland, over 23,000 miles.