9 years of oxygen-depleted summers: Ocean off Oregon has disturbing pattern
For the past nine years, every summer the oceans on the continental shelf off Oregon has been drastically depleted of oxygen. This was first measured by Francis Chan and Jack Barth of Oregon State University in 2002. In 2006 the water showed no detectable oxygen for four weeks and the anoxic zone covered up to 3,000 square kilometers.
Unlike other oceanic dead zones, the cause of the hypoxic conditions off the Oregon coast is not from excess fertilization upriver. In a news feature, “Dead in the Water,” published today by Nature, Virginia Gewin examines a possible cause for this event that has wider implications. And with increasing water temperatures and accompanying alterations of currents, it is likely that subsurface hypoxic patches are losing more oxygen and growing larger.
Chan and Barth propose a local explanation for the hypoxia off the Oregon coast. Normally there are late winter-early spring storms which cause upwelling of deeper waters (with lower oxygen and higher mineral content) which mixes with coastal waters. It used to be that there was enough relenting of the storms to allow the coastal waters to sufficiently mix with oxygenated waters to restore oxygen levels. Recently, however, the storms have not relented enough. Then during warmer months, the below normal oxygen water (with higher mineral nutrient content) permits large blooms of phytoplankton, which, when they die off, are decomposed by oxygen-fueled microorganisms which deplete the already low oxygen water of its remaining oxygen.
The problem is that throughout the world hyoxic zones have been increasing. Waters at 600-1,200 meters are generally permanently oxygen limited because they have little mixing with higher oxygen upper waters. The lower level is called an oxygen minimum zone (OMZ). Chan and Barth have reviewed measurements over 30 years and have discovered that the water above the OMZ has been losing oxygen. And around the world OMZs have been losing oxygen and expanding vertically and horizontally. It is not possible, owing to scarcity of historical recordings worldwide to determine if the loss of oxygen is related to warming. But there is some concern that if currents slow because of warming (as they are expected to do), existing OMZs will become larger.