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Hood Canal is a fjord off the Puget Sound. The [{main focus on the thesis}] discusses different contributing factors that [{immenently influence}] low-oxygen conditions (hypoxia) in Hood Canal. One overriding factor is the underwater topography of the canal. The deepest parts of the canal are more than 600 feet (180 m) deep, but at the entrance is a sill that is only 150 feet (46 m) deep. Inlets or bays [{within}] this fjord often experience sluggish water exchange. The Hood Canal Dissolved Oxygen Program (HCDOP) and the United States Geological Service (USGS) are studying Hood Canal circulation, trying to model the tidal circulations and salinity distribution patterns between the canal and Admiralty Inlet. Other factors that, when combined with the constrictive shape, could also influence hypoxia in Hood Canal are:

  1. Pacific Ocean marine water may be entering with a lower oxygen content than historically received
  2. Marine water may be entering at a density lighter than needed to flush out Hood Canal quickly or effectively
  3. Riverine freshwater input as changed—increased or decreased—altering the stratification of Hood Canal and have competing effects on marine water input
  4. Organic material input may have increased
  5. Light input may have increased (algae growth increases with light, as well as organic nutrient input)
  6. Wind currents may have altered water column circulation

The picture surrounding hypoxia in Hood Canal is complex; research models point to more than one contributing factor. Nutrient level is a large issue due to the human impact. The supply of nutrients, primarily nitrates, to the euphotic zone is thought to impact levels of dissolved oxygen. Nutrients feed algae, which under the right conditions, "bloom" and then die and decompose; the entire process requires a large amount of oxygen. This decreases the oxygen in the water column, lowering the dissolved oxygen level.

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.

Another factor mentioned by the HCDOP is the influence of the ocean water. The ocean water that enters Hood Canal is like most estuaries: fresh, warm water flows 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.

Finally, seasons play a role. The warmer temperatures, longer days, and lower winds change flushing conditions. Low oxygen conditions are at their 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.

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