Millions of dead crabs are washing up onto Oregon and Washington state beaches from the offshore “dead zone”.
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. This carnage was easily visible to a team of research scientists from Oregon State University, who sent an underwater vehicle, equipped with video cameras, into the depths to look around.
“We saw a crab graveyard and no fish the entire day,” noted Jane Lubchenco, co-author of the papers that report on their discovery. Lubchenco is the Valley Professor of Marine Biology at Oregon State University.
“Thousands and thousands of dead crab and molts were littering the ocean floor, many sea stars were dead, and the fish have either left the area or have died and been washed away.”
The team measured the dissolved oxygen in these dead areas and made a shocking discovery: there was almost none at all. When dissolved oxygen is 1.4 milliliters per liter, it is considered hypoxic for most marine life — so a “dead zone” forms. However, some of the data collected by the team from one area off Cape Perpetua on the central Oregon coast showed that dissolved oxygen was as low as 0.5 milliliters per liter in just 45 feet of water; 0.08 in 90 feet; and 0.14 at 150 feet depth. Data collected from other areas off the Oregon coast are similar (figure 1);
Figure 1: Dissolved oxygen profiles during the upwelling season (mid-April to mid-October) in the upper 800 m of the continental shelf and slope of Oregon (42.00°N to 46.00°N). (A) 1950 to 1999 from the World Ocean Database and Oregon State University archives (n = 3101 hydrocasts, blue). (B) (A) with additional data for 2000 to 2005 (n = 834 hydrocasts, green). (C) (A) and (B) plus data for 2006 (n = 220 hydrocasts, red). The black vertical line denotes the 0.5 ml l-1 threshold. (Insets) Overlapping locations of hydrographic (blue, green, and red) and remotely operated vehicle (black) stations through time and the 100-m and 1000-m isobaths. [larger view of data graphs].
Oxygen concentrations that low have never before been measured off the U.S. West Coast (figure 1A).
These low-oxygen “dead zones” have suddenly been appearing along various coastal regions throughout the world recently and result from a variety of causes. For example, 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. As the bacteria break down the dead algae and other microscopic plants and animals, dissolved oxygen is removed from the seawater, thereby creating a low-oxygen “dead zone” where most creatures cannot survive.
According to scientists, 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 — sometimes there is none at all. This causes every living thing in the area to either die or flee, further adding to the ecological imbalance.
Unfortunately, this cycle has repeated itself every summer and autumn ever since those Oregon crab fishermen first noticed its effects in 2002. Neither El Nino nor La Nina have any demonstrable effect on this phenomenon. This particular dead zone represents one of the many ways in which climate change is damaging the global environment: by depleting the concentrations of dissolved oxygen in a benthic marine habitat, much larger marine communities that cannot adapt quickly enough are also severely disrupted.
“We seem to have crossed a tipping point,” Lubchenco observed. “Low-oxygen zones off the Northwest coast appear to be the new normal.”
This paper was published in Science.
Chan, F., Barth, J.A., Lubchenco, J., Kirincich, A., Weeks, H., Peterson, W.T., Menge, B.A. (2008). Emergence of Anoxia in the California Current Large Marine Ecosystem. Science, 319(5865), 920. | DOI: 10.1126/science.1149016 [PDF]. (story and data figure).
LATimes (quotes and “dead zone” graphic).