Nitrogen and phosphorous-laden sediments flowing into Lake Erie from inland sources are a primary cause of oxygen-starved dead zones. This animation shows sediment plumes in Lake Erie during March 2010.
Areas of low oxygen are 30 times more prevalent in the nation’s waterways now than they were in 1960, according to a recent federal report. And climate change means they will continue to worsen.
The multi-agency report [PDF, 2.7 MB], released in September, states that the low-oxygen condition known as hypoxia has been detected in half of the more than 600 national waterways analyzed by the National Oceanic Atmospheric Administration, the U.S. Geological Survey, the U.S. Environmental Protection Agency, and the U.S. Department of Agriculture.
Hypoxia is often caused by increased nutrients such as phosphorous and nitrogen. These common components of fertilizer often enter lakes and streams with runoff from farms and lawns. Nitrogen released into the atmosphere from fossil fuel combustion can also settle onto the water’s surface.
“What we do affects the amount of nutrients that wash off the land and are ultimately carried by rivers to the coast,” said report co-author Herb Buxton, coordinator of the Geological Survey’s program on toxic substances hydrology. “Runoff is the conveyer belt for moving nutrients from the landscape to our coastal systems.”
The nutrients cause algae to grow. When the algae dies, the bacteria that decompose dead algae also consume oxygen. The resulting low oxygen in the water typically kills the animals in the area or forces them to move. This process is called eutrophication, and the resulting low-oxygen regions are known as dead zones.
Hypoxia has been present across the U.S. for more than 50 years. Detected in the Chesapeake Bay in the 1950s. it has since been found in the Gulf of Mexico, Lake Erie, Lake Michigan, and off the coasts of Washington and Oregon.
Globally, hypoxia has increased ten-fold in the past 50 years, the report states.
Lake Erie has been affected by hypoxia for decades. Erie receives the highest sediment loads of all the Great Lakes, according to the report, mainly from the Maumee River in northwestern Ohio.
According to the EPA, source control and limiting the use of phosphorus in the 1970s helped to practically eliminate hypoxia from Lake Erie by the mid-1980s. However, it has been progressively increasing again for the past 20 years.
The growth of hypoxia in the Great Lakes over the past few decades has been small compared to many other regions. The report notes that most of the increases hypoxia levels have been in the North Atlantic, South Atlantic and Pacific.
But climate change will make things worse, researchers say.
The warmer the water, the harder it is for it to absorb oxygen. And shifts in precipitation associated with climate change will also increase nutrient-laden runoff. “Many scientists believe that climate change will result in wider swings in climate, like wetter and drier years,” Buxton said. “The past record has shown us that wet years, particularly following dry years, have high nutrient yields and correspondingly larger hypoxic zones.”
Similarly, extreme levels of rainfall will cause more soil erosion and allow for higher levels of phosphorous to enter a body of water. While phosphorous is a naturally occurring compound in soil, it is particularly abundant in over-fertilized soil.
The effects of hypoxia on coastal waters are wide-reaching both ecologically and economically. Bottom-feeding fish and shellfish are often killed in hypoxic conditions. This greatly alters the ecosystem, since the loss of these creatures affects the amount of food that exists for larger predators or others in the food chain.
“Many organisms cannot live in areas that have low oxygen,” said report co-author Libby Jewett, hypoxia research program manager for NOAA. “If there is anything living on the bottom such as oysters, hypoxia will certainly kill them.”
Although federal research has focused on hypoxia for decades, the report concludes that more is needed, since the condition has continued to worsen. “There is a lot to be figured out about the interaction between climate and hypoxia,” says Jewett. “We don’t really know how that [climate change] will affect each and every one of the water systems, but it most likely will.”
This post was originally published by Great Lakes Echo, a project of the Knight Center for Environmental Journalism at Michigan State University.
