Q: What is a dead zone?
A: ‘Dead Zone’ is a popular term for areas of low oxygen in coastal waters where the dissolved oxygen levels are too low to support typical marine life such as fish, shrimp, crab, snails, clams, or sea stars. The scientific name is hypoxia or low dissolved oxygen. In areas where the oxygen concentration is too, low fish, shrimp, and crabs that can will move out of the area. Organisms such as burrowing crabs, burrowing shrimp, worms, brittle stars and other sediment organisms cannot escape and will die as the oxygen levels continue to decrease. The low oxygen area thrives with microbial communities. The term ‘dead zone’ is somewhat a misnomer because the upper water column is full of marine life, schooling fishes, where the oxygen levels are sufficient to support them.
Q: What, if anything, can be done to stop the growth of dead zones? How can organic agriculture assist in this process?
A: The solution to the explosion of ‘dead zones’ around the world is control of excess of nutrients, primarily nitrogen and phosphorus, that enter into the waters of the estuaries and the coastal waters. The nutrients come from agricultural practices, urban runoff, atmospheric deposition, industrial and municipal waste. The proportion of these inputs depends on the watershed. In the instance of the Mississippi River watershed and the Gulf of Mexico hypoxic zone, most of the nutrient inputs are from agricultural purposes. This means that agricultural nitrogen and phosphorus need control, at least in the Mississippi River watershed. In Long Island Sound, most of the excess nutrients come from municipal waste water, urban runoff and atmospheric deposition.
Activities that encourage organic agriculture that reduce the amounts of artificial fertilizers should decrease the amounts of fertilizers that enter the Mississippi River watershed and the Gulf of Mexico. This will lead to fewer water quality problems in the watershed, such as harmful blue-green algal blooms and high nitrates in drinking water, and hypoxia and noxious and harmful algal blooms in the Gulf of Mexico.
Q: What are the primary causes of dead zones? Are they naturally occurring or are they man-made?
A: Some areas of low oxygen in the ocean are natural and formed from the decomposition of organic matter at depth where a density discontinuity forms naturally. These are the oxygen minimum zones of the open ocean. ‘Dead zones,’ or areas of low oxygen, are formed at the intersection of physically controlled systems and high biological productivity. An area of low oxygen will form only when there is a physical stratification of the water column. This means a lower salinity, higher temperature water on the surface compared to a higher salinity and cooler water column on the bottom. This is most likely to occur off areas of higher freshwater discharge, such as rivers entering estuaries or the coastal ocean.
The fresh water brought to the coastal or estuarine system is loaded with phosphorus and nitrogen, which stimulate the production of high phytoplankton biomass. These phytoplankton are incorporated into the food web or die and sink into the bottom waters. Even those phytoplankton that enter the food web are often taken up into zooplankton that then expel fecal pellets that carry the undigested phytoplankton to the sea bed. The high organic carbon that reaches the bottom from high production in the surface waters forms a pool of organic carbon that is decomposed by aerobic bacteria that consume oxygen in the process. The oxygen consumed by the bacteria is depleted at a much higher rate than the resupply from the surface waters. The number of human-caused areas of low oxygen around the coastal ocean has increased since the 1960s and continues to increase.
Q: What are some examples of places in the United States which dead zones can be found?
A: Low oxygen areas are typical of many estuaries where stratification of lower salinity and higher salinity waters occurs combined with high nutrient inputs that stimulate the growth of phytoplankton that eventually sink to the bottom and are decomposed by bacteria that consume oxygen in the process. The largest area in U.S. waters is the low oxygen off the Mississippi River on the northern Gulf of Mexico, where the hypoxic zone encompasses up to 22,000 square kilometers of low oxygen bottom waters, equal to the size of the state of New Jersey. Similar but smaller areas occur in the Chesapeake Bay, Long Island Sound, the Neuse River estuary and Pamlico Sound, Mobile Bay, parts of Pensacola Bay, Galveston Bay, outlets of many estuaries, San Diego Bay, the inner continental shelf of Oregon and Washington, and Puget Sound. And many others. The increasing dead zones off Oregon and Washington are attributed to shifts in ocean currents that bring colder, lower oxygen water from depth into shallower waters.
