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==Hood Canal Hypoxia==
The Hood Canal is a long, narrow and deep fjord-like body of water. There are several factors that lead to low-oxygen conditions ([[hypoxia]]) in Hood Canal. First is the underwater topography of the Canal. The deepest parts of the Canal are more than 600 feet deep, but at the entrance is a sill that is only 150 feet deep. This sill leads to sluggish water exchange between Admiralty Inlet and Hood Canal. Water exchange from tides is also sluggish in Hood Canal due to its shape and depth. Second is the supply of nutrients, primarily nitrate, to the euphotic zone. There are both natural and man-made sources of nutrients. The primary natural source is in ocean water that flushes Hood Canal. Man-made sources include leaking septic systems, storm water runoff, agriculture and various other sources. The presence of nutrients leads to algae growth, which consumes oxygen when the algae die and decompose, contributing to the low oxygen conditions in these waters. Third is the source of ocean water that enters Hood Canal. Circulation in Hood Canal is like most estuaries: fresh, warm water flow out at the surface and is replaced by cold, salty water at depth. The cold, salty ocean water that enters Hood Canal comes into Puget Sound from the open ocean and has not recently been in contact with the atmosphere. As a result, this water is initially somewhat depleted in oxygen.
Low oxygen conditions are at there worst in the late summer, after several months of limited flushing and maximum plankton production near the surface. In some years, oxygen becomes sufficiently depleted that animals cannot survive. These kills may occur either locally or over a wide area. These issues are part of a current research program (Hood Canal Dissolved Oxygen Program).
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