For the sixth straight year now, a large expanse of oxygen-depleted water — known as a "dead zone" — has appeared off the coast of central Oregon, raising concerns among scientists about the long-term health and biodiversity of the ocean ecosystem. This phenomenon occurs when northerly winds move the top layer of ocean water out to sea, pulling up nutrient-rich, oxygen poor water from the deep ocean to replace it.
The rich concentrations of nutrients help stimulate large plankton blooms and subsequent die offs, which result in bacteria eating the decomposing plankton and consuming what little oxygen remains in the water. Though fast swimming fish are sometimes able to escape these dead zones, other slower moving organisms — such as crabs, sea stars and other invertebrates — die by suffocation. Fish that are caught by the low oxygen levels suffer a host of debilitating reproductive problems, including lower egg counts, lack of spawning and the decreased size of reproductive organs. The dead zone's return was discovered by oceanographers at Oregon State University, who deployed robotic underwater gliders and other monitoring devices over the past few months to assess oxygen levels in the water.They discovered that oxygen levels on reefs previously devastated by past dead zones had dropped to 0.5 mL/L by the end of June — a far cry from the 1.4 mL/L level considered to be hypoxic for most marine life. According to Francis Chan, a marine ecologist at OSU, this recurrent dead zone is just another sign of global warming's harmful impact on ocean ecosystems around the world — prompted by unprecedented changes in wind patterns and water currents.
The fear now is that increasing global temperatures and sea level rises could trigger ever stronger winds and — in turn — larger dead zones off the coast of Oregon and other states in the near future.
Via ::ScienceNOW: "Dead Zone" Reappears off Oregon Coast (news website)
See also: ::Indian Ocean Shows Signs of Global Warming, ::Giant Ocean Waves Getting Speedier, Courtesy of Global Warming
Image courtesy of National Geographic