Low-oxygen scenario following unusual course this year in Hood Canal

Death came early to Hood Canal this year, demonstrating just how odd and unpredictable ocean conditions can be.

Fish kills caused by low-oxygen conditions in southern Hood Canal usually occur in late September or October. That’s when low-oxygen waters near the seabed are pushed upward by an intrusion of heavier water coming in from the Pacific Ocean and creeping along the bottom. Winds out of the south can quickly blow away the surface waters, leaving the fish with no escape.

That’s basically what happened over the past month, as conditions developed about a month earlier than normal. South winds led to reports of fish dying and deep-water animals coming to the surface to get enough oxygen, with the worst conditions occurring on Friday. Check out the video on this page by Seth Book, a biologist with the Skokomish Tribe, who found deep-water ratfish swimming near the surface.

The story of this year’s strange conditions actually begins about a year ago and involves a 1,000-mile-long “blob” of unusually warm ocean water off the West Coast. State Climatologist Nick Bond, who coined the term “blob,” explains its formation in an article in Geophysical Research Letters with a summarized description by Hannah Hickey in UW Today.

The warm, low-density coastal waters related to the blob came into Hood Canal on schedule last fall, but they were not dense enough to flush out the low-oxygen waters, according to University of Washington oceanographer Jan Newton.

Hood Canal entered 2015 with the least-dense waters at depth over the past 10 years. They remained in a hypoxic state, meaning that levels were below 2.5 parts per million. Sea creatures unable to swim away can be unduly stressed and unable to function normally at that level. Conditions worsened into the summer, when the hypoxic layer at Hoodsport grew to about 300 feet thick.

By then, the annual intrusion of deep seawater with somewhat elevated oxygen levels was on its way into Hood Canal, spurred on by upwelling off the coast. This year’s waters are more normal in density, though their arrival is at least a month early. By August 9, the hypoxic layer at Hoodsport was reduced from 300 to 60 feet, pushed upward by the denser water.

It’s always interesting to see this dynamic play out. The layer of extreme low-oxygen water becomes sandwiched between the higher-oxygen water pushing in from the ocean and the surface water, which ordinarily stays oxygenated by winds and incoming streams. Without south winds, the middle low-oxygen layer eventually comes up and mixes into the surface layer.

If south winds come on strong, however, the surface layer is blown to the north, causing the low oxygen water to rise to the surface. Fish, shrimp and other creatures swim upward toward the surface, trying to stay ahead of the rising low-oxygen layer. When the low-oyygen layer reaches the surface, fish may struggle to breathe in the uppermost mixing layer. Unfortunately, the fish have no way of knowing that safer conditions lie down below — beneath the low-oxygen layer and within waters arriving from the ocean.

Jan Newton reported that the low oxygen levels in southern Hood Canal earlier this year were the most extreme measured over the past 10 years. So far, however, the fish kills don’t seem as bad as those in 2003, 2006 and 2010, she said.

The graph below shows how the deep layer coming in from the ocean at 279 feet deep contains more oxygen than the middle layer at 66 feet deep. The surface layer, which normally contains the most oxygen, dipped to extremes several times near the beginning of August and again on Friday, Aug. 28. These data, recorded from a buoy near Hoodsport, are considered unverified.

Graph

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