Dead zones in the ocean are becoming larger, and more pronounced due to the effects of global warming, according to a new report from the Smithsonian Tropical Research Institute and the Smithsonian Environmental Research Center.
Hypoxia, a reduced concentration of oxygen in oceans, creates dead zones in oceans around the world. One of the largest of these regions forms in the Gulf of Mexico each year during the spring. These areas can form naturally, but chemical runoff from farms is the driving force behind many such regions, and are the primary concern of environmentalists. High concentrations of nitrogen, phosphorus, and other chemicals feed algae, which consume oxygen needed by other species.
"Excess nutrients that run off land or are piped as wastewater into rivers and coasts can stimulate an overgrowth of algae, which then sinks and decomposes in the water. The decomposition process consumes oxygen and depletes the supply available to healthy marine life," NOAA officials reported on their Web site.
Global warming is expected to raise temperatures by an average of 3.6 degrees over 94 percent of the global ocean during the next few decades. Warmer water is able to hold onto less oxygen than cooler water, which means that hypoxia could be more likely to occur as global warming increases the average temperature of waterways.
Deep underwater, these areas can dissipate as surface water, rich with oxygen, sinks, refilling the region with the life-giving gas. This process, driven by the sinking of cold water, could become less common as global temperatures rise, leading to longer lifespans for the oxygen-poor areas.
Dead zones, like the one that forms every year in Chesapeake Bay, can last for months in deeper waters. Such regions can form temporarily in shallow water, lasting just overnight, before oxygen returns to the water.
Hypoxia is becoming more common, as the number of recognized regions has doubled each decade for the last 50 years.
"They're having a big impact on life in the coastal zone worldwide. A lot of people live on the coast, and they're experiencing more fish kills and more harmful algal blooms. These are effects of dead zones that have an impact on our lives," Keryn Gedan from the the University of Maryland and marine ecologist at the Smithsonian Environmental Research Center, said.
One possible positive effect of rising global temperatures could be a higher population of shrimp and other crustaceans, which could eat some algal blooms, mitigating the impact of rising temperatures on marine life.
Study of the effect of global climate change on oceanic dead zones was profiled in the journal Global Change Biology.
