Ever since it was first noticed by crab fishermen who hauled up hundreds of dead and dying crabs in 2002, the “dead zone” that popped up in the waters along the northwestern coastal shelf just off the coast of Oregon has claimed unknown millions of lives. This oxygen-depleted region has transformed formerly rich seafloor communities teeming with life into vast graveyards filled with the bodies of crabs, echinoderms, molluscs, sea worms and other creatures.
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a low-oxygen zone appears each spring off the coast of Louisiana due to fertilizers in farm runoff and sewage present in the Mississippi River. When the Mississippi flows into the sea, it creates a nutrient-rich area that triggers huge but short-lived algal blooms that soon die, sink to the seafloor and are decomposed by bacteria that produce toxic sulfide gases.
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the dead zone off the West Coast of North America has another cause: global warming. Here’s how it works: Winds cause the oceanic rivers of nutrients, such as the California Current in this case, to flow upwards from the deep, carrying nutrients and phyoplankton into the sunlight, which triggers the phytoplankton to reproduce, to “bloom”. This is the normal state of things, but since global warming has been causing land temperatures to increase, these winds have become stronger and more persistent. This is not normal because it prolongs the oceanic upwelling, producing a surplus of phytoplankton that isn’t consumed and subsequently dies, and sinks to the seafloor to decay. As the bacterial-mediated breakdown occurs, dissolved oxygen in the surrounding water is depleted to dangerously low levels