Topics Overview
Hypoxia
Hypoxia, defined as dissolved oxygen (DO) concentrations less than 2 mg / L, has become widespread throughout estuaries and semi-enclosed seas throughout the world (Diaz 2001). While hypoxia may be permanent or intermittent, it is most commonly manifested as a seasonal disturbance, appearing in mid- to late summer after vertical stratification prevents replenishment of deep water DO. The duration, extent and magnitude of seasonal hypoxia has dramatically increased over the past few decades in response to anthropogenic eutrophication (Diaz and Rosenberg 2008) and is now a common and regular feature in marine ecosystems that have strong vertical stratification and low flushing rates. Additionally, climate change may be altering the frequency and intensity of hypoxic conditions in coastal ecosystems (Chan et al. 2008).
Hypoxia is an important concern because low dissolved oxygen can have direct and indirect effects on marine communities and natural resources. Hypoxia and anoxia can be lethal to animals when oxygen levels are depleted beyond species physiological tolerances. For sessile organisms who have limited capacity to seek out refuges from hypoxia, direct lethal impacts may be most severe (Diaz and Rosenberg 1995). Mobile species generally act to minimize exposure to low DO through distributional shifts to refuges that have higher DO levels. While these responses minimize direct lethal impacts of low DO, they can induce indirect ecological effects such as reduced feeding rates, enhanced vulnerability to predators and reduced growth rates (Breitburg 1992, Breitburg et al. 1997, Eby and Crowder 2002, Bell et al. 2003, Craig and Crowder 2005, Aumann et al. 2006).
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Additional resources:
Hood Canal Dissolved Oxygen Program
Independent review of human impacts on dissolved oxygen in Hood Canal
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There are at least 28 species of rockfish in the Salish Sea, but their populations have declined in the past several decades. The proceedings from a 2011 rockfish recovery workshop in Seattle are now available.
An independent review conducted by the Puget Sound Institute is featured in findings by the Environmental Protection Agency and the Washington State Department of Ecology that there is currently “no compelling evidence” that humans are the cause for recent trends in declines in dissolved oxygen in Hood Canal.
Puget Sound circulation is driven by tidal currents, the surface outflow of freshwater from Puget Sound rivers, and deep inflow of saltwater from the ocean; and is influenced by wind strength and direction. Tidal currents dominate the circulation, and typically a two-layered pattern of estuarine circulation is superimposed on the tides. Deep, dense saltwater enters Puget Sound from the Strait of Juan de Fuca through Admiralty Inlet, part of it flowing south into the Main Basin and part flowing north up into Whidbey Basin. The resulting landward-flowing water replaces the bottom water of Puget Sound and keeps it from becoming stagnant, and the out-flowing surface water flushes Puget Sound.