Aquatic dead zones are bodies of water where the oxygen conditions are so low that marine life is unable to be supported. The condition, known as hypoxia, occurs when dissolved oxygen (typically below 2 mg/l) is below the threshold needed to sustain life and the water becomes a biological desert.
Aquatic dead zones can occur naturally although a growing number of dead zones observed are the result of human activity.
The dramatic increase in hypoxic waters is attributed to nutrient pollution and fossil fuels according to a review paper (SCIENCE Aug 15 2008) by Robert Diaz of the Virginia Institute of Marine Science, and Rutger Rosenberg of the University of Gothenburg. Fertilizer laden water runs off from agricultural fields and into the water system is responsible for a dramatic increase in planktonic algae growth in coastal areas.
The algae die off and are decomposed by microbes which depletes available oxygen in deep water areas, creating a dead zone. This process is known as eutrophication. Nitrogen entering the atmosphere from fertilizers and fossil fuels also contributes to this process when the nitrogen is reintroduced into the water cycle through rain and snow.
World Map of Aquatic Dead Zones
There are now over 405 documented dead zones around the world, including 166 across the United States. The darker brown areas in the map show areas of denser human population which correlates with the occurrence of dead zones downriver from higher populations.
Seasonal Aquatic Dead Zones
Dead zones can occur seasonally.
A dead zone has occurred off the coast of Oregon each summer since 2002. Each year a northerly wind pushes surface waters away from the coast which in turn gives rise to an upwelling of colder and less oxygenated water that originates from deeper in the ocean.
This water source is also dense in nutrients which allows for macrobiotic activity which further depletes the oxygen poor environment, creating the dead zone.
“These high-nutrient, low-oxygen waters pack a one-two punch in creating dead zones,” says Francis Chan, an Oregon State University scientist studying the annual dead zone event. “Within 1 kilometer of the shore, there are places where oxygen levels are near zero.”
In 2011, NASA reported on the seasonal occurrence of dead zones in the Chesapeake Bay area in Maryland was exacerbated by an increase in runoff due to Tropical Storm Lee:
In 2011, the Maryland Department of Natural Resources recorded low oxygen levels in 30 to 40 percent of the Maryland portion of the Bay—a much larger dead zone than average. Hurricane Irene stirred the water and dissipated the dead zone, but the run-off from Lee may trigger a second eutrophication event. Fortunately, the extreme runoff occurred in early September, when the cooling water will mix more easily and push oxygen into the dead zones. However, the nutrient-filled runoff from Tropical Storm Lee may seed the development of next summer’s dead zone.
The largest dead zone in North America is found in the Gulf of Mexico. The size of the dead zone in the Gulf of Mexico was measured at 8000 square miles in 2008 (More: Gulf of Mexico Remains the Second-largest Low-Oxygen Dead Zone on Earth).
Diaz, R. J., & Rosenberg, R. (2008). Spreading Dead Zones and Consequences for Marine Ecosystems. Science, 321(5891), 926-929.
Overview of Hypoxia and Nutrient Pollution. N.d. Center for Sponsored Coastal Ocean Research.