Oxygen-depleted dead zones are extending around the coastal regions of oceans — both here in Virginia and around the world — at an alarmingly swift rate, according to a recent study published by Virginia Institute of Marine Science professor Robert Diaz.
VIMS is a graduate school of the College of William and Mary that focuses on marine science. It is located approximately 16 miles away from the main campus in Williamsburg.
In these hypoxic dead zones, aquatic life becomes unsustainable, and the unique biodiversity in each region is destroyed. The study, published in the journal Science, found that these dead zones have doubled every 10 years since 1960.
Diaz reported that over 50 percent of U.S. estuaries and bays have reported dead zones; between Williamsburg and Washington, D.C., 15 separate dead zones have been identified. The second-largest dead zone on earth exists in the northern Gulf of Mexico.
Dead zones are caused by nitrogen-rich agricultural runoff, which travels through tributary rivers and feed in to the ocean’s coastal region. The nitrogen causes photosynthetic plankton to grow on the surface of the water. The plankton eventually decays and sinks to the bottom of the ocean, where they are decomposed by oxygen-consuming microbes. This depletes a vast majority of the area’s oxygen supply, making life unsustainable for several species and altering the food chain within the area.
“Too much production of organic matter is part of what causes a dead zone,” Diaz told The Flat Hat. “If the bottom water is isolated from the surface supply of oxygen, hypoxia results and a dead zone forms. The leading cause of the overproduction of organic matter in the sea is land runoff of nitrogen, most of which is coming from agriculture.”
Low oxygen levels are responsible for eliminating several fish and crustacean species from the bottom waters, leaving behind little more than microbes and stripping away the area’s biodiversity.
“All dead zones lower biodiversity by eliminating sensitive species,” Diaz said. “The most common response of the invertebrates that cannot escape is to die.”
All of the dead zones studied by Diaz and his colleagues were caused by human activity, although other areas have similar problems that are not the result of human actions or interference.
“There are also large areas of low oxygen in the oceans that are called oxygen minimum zones; these are naturally occurring and
typically in deep water,” Diaz said.
The naturally existing dead zones are prevalent in lakes, and although they do not naturally pose a threat to oceanic health, human activity frequently worsens their conditions.
“Lake Erie has the second largest dead zone in the U.S., but Lake Erie is prone to [the] development of low dissolved oxygen,” Diaz said. “However, it is clear that human activity has greatly expanded the size of the dead zone there.”
In order to stop the spread and development of dead zones, it is imperative that fertilizers remain on the land and that agricultural runoff is prevented. Diaz is optimistic that current dead zones can be revived and will once again be sustainable.
“Almost all systems that have reduced nutrient inputs have recovered from having a dead zone,” he said. “Many small systems improved in the 1980s from sewage treatment upgrades, like Delaware River, Hudson River and the East River.”
Diaz said he will continue his research into oxygen levels in the world’s oceans.
“Next we need to estimate the effects on fisheries and make recommendations on how to reverse these effects,” he said. “If dead zones continue to spread, there will be very serious consequences to our living resources.”