In recent years, Hood Canal has become a poster child for the perils facing Puget Sound. A 2003 report by the Pew Oceans Commission listed it and Los Angeles harbor as the only two dead zones on the Pacific Coast. But that's not the whole story. Just recently, scientists reported finding more than 400 other dead zones in salt water around the globe. The Seattle Post-Intelligencer reported on August 15 that "dead zones with too little oxygen for life are expanding in the world's oceans." The P-I referred to an article in Science showing that the number of dead zones in the world had doubled what the United Nations had reported in 2006. According to the P-I, researchers maintained that "some of the increase is because of the discovery of low-oxygen areas that may have existed for years and are just being found [...] but others are actually newly developed."
Hood Canal's infamous dead zone has killed thousands of fish on four different occasions in the past decade. "Hood Canal Fish Suffocate" blared a P-I headline in the fall of 2006. Robert McClure reported:
Scientists are scrambling to document what appears to be the most widespread fish kill to date in Hood Canal, the deep and poorly flushed waterway that researchers say suffers from oxygen levels at their lowest ebb in at least five decades [...] Reports from area residents started early Tuesday of dead shrimp, Dungeness crab, lingcod, flounder, sand lance and other fish. The kill stretches about six miles along the 'Great Bend' of the 60-mile-long, glacier-carved fjord.
This spring, thousands of Hood Canal's tiny, shrimp-like krill died.
Dead zones have been expanding near river mouths, where agricultural fertilizer feeds plankton blooms. Other dead zones — such as one that lies deep in the Black Sea — can be traced to a lack of mixing between oxygen-poor deep layers and the better-oxygenated water near the surface. In effect, the deck doesn't get shuffled, so the low-oxygen cards stay on the bottom.
McClure's 2006 P-I story noted that the sources that are "thought to be contributing the nitrogen that drives Hood Canal's algae blooms are fertilizers, leaking septic tanks, ocean water and alder trees that pop up on vast patches of clearcut forest."
That's all true, but notice the order of causes. Fertilizer and septics come first, then ocean water and alder. This, like a lot of other rhetoric about Hood Canal, helps feed our well-earned guilt about what we have done to Puget Sound. However, it doesn't fit the facts.
Most of the nitrogen entering Hood Canal has nothing at all to do with us. And yet, arguably, in a political context, our own tiny contribution is the only small piece of the large puzzle that really matters. As scientists conducting the Hood Canal Dissolved Oxygen Program Integrated Assessment and Modeling Study reported at the end of June:
Several natural factors, such as sunlight, wind, and external ocean conditions, affect Hood Canal oxygen conditions substantially and are the major factors in the interannual variation of 1-2 mg/L in minimum oxygen concentrations. Both the mainstem of Hood Canal and Lower Hood Canal are affected by these factors. However, we also find that human factors can be of sufficient magnitude to reduce minimum oxygen concentrations on the order of 1.0 mg/L in Lower Hood Canal over the course of the summer.
The layer of dense, cold water that has been lying on the bottom of Hood Canal will soon rise toward the surface. This water will be low in the oxygen that fish need but high in nitrogen. If it rises all the way to the surface, fish will have no place to go, and they will suffocate. After the Hood Canal fish kill in 2006, McClure reported, "Low-oxygen water in the bottom of the canal quickly surged up to overtake fish [...] Scientists were able to measure it because of a major new scientific investigation started after fish kills in 2002, 2003 and 2004."
There's more than one way to suffocate a fish. Toward the surface, that nitrogen-rich water from the depths meets additional nitrogen from the land, most of it from rotting alder leaves. (Originally, alders weren't much of a factor, but as people cut the native conifers, alders sprang up in the clearings.) Some additional nitrogen leaches from septic drain fields, and lesser amounts come from fertilizer runoff and other artifacts of human population growth along the shores.
In water shallow enough for sunlight to penetrate, nitrogen feeds algae, which proliferate and then die. Bacteria eat them. Bacteria need oxygen, too, and they take it from the water. This process of "eutrophication" contaminated Lake Washington until METRO started dumping treated sewage from lakeside communities into Puget Sound. Take enough oxygen from the water, and a lot of fish are history.
Ocean circulation patterns drive that dense, high-nitrogen water into the Strait of Juan de Fuca. Heavier than surface water, it creeps along the bottom. When enough of it builds up in the main basin of Puget Sound, it slops over a relatively shallow underwater sill near Bangor and down into Hood Canal. The new flow of dense water coming into the canal drives dense, oxygen-poor water from other years up toward the surface. Some flows back out toward the Pacific. Some rises toward the surface. This happens in late September or October.
Clearly, around this time of year, there's not always enough oxygen to go around. That's when people start finding dead fish. But what really causes the oxygen depletion? "Can the human contribution of nitrogen be of sufficient magnitude to have an impact?" asks University of Washington oceanographer Jan Newton, who leads the dissolved oxygen study. Newton explains that the answer "really depends on when and where." In the main part of Hood Canal, the answer is no. But, Newton says, "in lower Hood Canal, in late summer, the answer is yes." She explains that below Hood Canal's Great Bend, where the water is shallower and flushes less, if weather conditions are just right (or just wrong) in late summer, then yes, nitrogen from septic tanks and other anthropogenic sources "could tip the balance" toward oxygen concentrations at which fish expire. Something else killed the krill, she says, but septics — although they contribute far less nitrogen than mother nature — may be among the reasons for fish kills.
Retired University of Washington oceanographer Alyn Duxbury is skeptical. Duxbury, a co-author with his wife of Fundamentals of Oceanography, says
The problem time is in late September or October when the big final surge of deep water from the coastal area finishes filling the Main Basin of the Sound and then spills over the Hood Canal sill to displace the resident deep water upward in the end of the Canal ... The problem becomes as follows; small inter-annual changes in a very large natural nutrient flux system, in late summer and early fall, can actually cause a change in the nutrient budget that is many times more significant than the nutrient flux emanating from the on-site septic systems. How can one point to the small nutrient influx from the septic systems and say it is the factor that pushes the problem over the top, when inter-annual changes in natural fluxes are magnitudes larger?
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