Hood Canal - Low Oxygen Levels

Low Oxygen Levels

September 2006 marked the discovery of the largest dead zone in the history of Hood Canal. The dead zone may have been caused by low oxygen levels due to algal blooms. Algal blooms occur in part because of warm weather and the slow turnover of water in the southern end of the canal, causing the build-up of nutrients from fertilizers and leaky septic systems. Organic matter, brought in by ocean water and certain trees, could additionally be contributing to the high nitrogen levels in the basin. Excess nutrients and organic matter causes a body of water to gain weight, through a process called eutrophication. In Hood Canal, eutrophication has led to unwanted algae blooms. Nitrogen combined with sunlight triggers algal growth. A lack of sufficient consumers has resulted in a mass overgrowth of algae in the basin. When the algae die, bacteria feed and their populations explode, robbing the water of oxygen. A state of hypoxia is created.

The fish kill may also be part of a natural 50-year cycle of oxygen levels in the canal, which has merely been influenced (but not controlled) by anthropogenic activity. Natural causes of hypoxia in Hood Canal include the timing of freshwater inflows, water layer stratification resulting from seasonal changes in surface temperature, and climate change. The oxygen level may also be partially due to a change in wind direction. The prevailing north wind generally pushes oxygenated water into the oxygen-depleted area. A sustained south wind will cut off this source of oxygen. Coastal upwelling from the Strait of Juan de Fuca bring in a surplus of nutrients into the Puget Sound, but fail to circulate oxygen through Hood Canal. Chronic hypoxia is observed year-round. This area of low-oxygen is often seen in Lynch Cove, but has been spreading towards the mouth of Hood Canal.

In May 2006, divers searching for invasive species discovered a mat of marine bacteria covering a 4-mile (6.4 km) stretch where all normal sea life was dead. The mat dissipated five months later. Jan Newton, oceanographer at the University of Washington, said it was important to note that Hood Canal has had very low oxygen for a long time. Similar mats have been found near Tacoma, Washington; San Diego, California; New York City; and New Orleans, Louisiana. Hypoxia and its detrimental effects on fish have been documented in Hood Canal since the 1970s. But hypoxia’s impacts on the area’s wildlife have been more significantly linked to growing urbanization along the coast. A recent study determined DO content, oxygen saturation, of the Sound’s Main Basin to consistently stay around 5 mg/l from the 1950s up through 2005. DO concentration in Hood Canal, however, was identified to have dramatically decreased from 5–6 mg/l in the 1950s to less than 0.2 mg/l in the twenty-first century. As of 2000, the National Research Council defined hypoxia to occur at a DO level of <2 mg/l—making Hood Canal’s current 0.2 mg/l composition far below life-sustaining conditions.

Hood Canal’s hypoxic state has had devastating effects on the sub-basin’s biodiversity. Critically low oxygen levels, due to increased bacterial growth, were observed in the lower portion of Hood Canal during the summer months of 2004 and 2005. A low dissolved oxygen content in Anna’s Bay and Lynch Cove is believed to have been responsible for the corresponding decline in spot shrimp catch by Skokomish Nation fishers within the same period of time. In 2010, there was a massive fish kill in the southern part of Hood Canal. Hundreds of fish and thousands of shrimp were found washed up onto the shore. Jan Newton, a local oceanographer, concluded that the water contained less than 1 millimeter per liter of dissolved oxygen and that such a low level is extremely stressful, often lethal, to the marine life in Hood Canal. The effects of Hood Canal’s hypoxic conditions are clearly seen by the public though massive fish kills, but it is important to identify other harmful impacts resulting from a lack of oxygen in the basin. Eelgrass beds, which are nurseries for salmon and crab, have declined more in Hood Canal than any other area of the Puget Sound. Bottom dwelling rockfish have also seen a decline due to a lack of eelgrass. Other notable cases of hypoxia and its adverse effects on biodiversity include the large-scale hypoxic zone that appears in the Gulf of Mexico each summer.

The Hood Canal Dissolved Oxygen Program (HCDOP), a partnership of 38 organizations, has been formed to combat the problem. This program will work with local, state, federal, and tribal government policy makers to evaluate potential corrective actions that will restore and maintain a level of dissolved oxygen that will reduce stress on marine life. The HCDOP-Integrated Assessment and Modeling study was started in 2005 to quantify marine processes and watershed loadings, assess biota-oxygen interactions, model key processes to measure drivers of oxygen, and to evaluate potential corrective actions. Government agencies, such as Puget Sound Partnership and the United States Geological Survey, have used HCDOP’s publically available information to conduct their own assessment and modeling studies of Hood Canal. Puget Sound Partnership is a Washington state agency responsible for protecting and restoring life in the Sound. The group is required to produce a “State of the Sound” report every two years. As of 2009, the Sound has recently shown signs of increased stress and degradation from human activity. A noticeable drop in spawning rates has had a negative effect on the ecosystem’s biodiversity and available habitats continue to decline. But the abundance of some species, like the Chinook salmon, have advanced and limited loadings of polycyclic aromatic hydrocarbon (PAHs) to Elliott Bay has improved water quality there.

Though difficult to regulate, a majority of proposed solutions have recommended the regulation of harmful, nearby anthropogenic practices. Human development has caused the most significant damage to Hood Canal’s ecosystem. The runoff from septic tanks dumps large quantities of nitrogen into the canal following each rainfall. The best way to solve this problem is to work with the community to create state of the art sewage treatment plants or, at the very least, upgrade old and damaged septic systems to prevent leaks. Several of Puget Sound’s counties have taken it upon themselves to create detailed on-site sewage codes. The 2010 King County Board of Health Code explicitly states that: “An owner may not allow an on-site sewage system or component or side sewer to remain in a condition of failure.” Because the Hood Canal region has experienced logging, as have the majority of watersheds around Puget Sound, the surrounding forests have become heavily degraded. This has caused nitrogen-rich Alder forests to leach excess nutrients into the water. Restoring wetlands along Hood Canal and restoring native plant life with low nitrogen impacts could help ameliorate this phenomenon. Since wetlands are great filters for the environment, this could prove a very effective way of lowering the impact of nitrogen on the basin. Coastal upwellings and tides are a huge issue for Hood Canal. Because of the lack of upwelling and tides pushing in oxygen rich waters into the canal, the area’s oxygen content suffers. Unfortunately, since the Puget Sound ecosystem is so large, it would be very difficult to oxygenate deeper waters. But surface-level DO could be improved by restoring natural flows of estuaries.

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