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Title: Average Dissolved Oxygen S. Mainstem
Author: Mark Warner
Data Source: Collias, UW, PRISM
File Size: 11 kb
File Type: Image (JPG or GIF)
Date Catalogued: Monday, June 25, 2007

This figure shows the average dissolved oxygen concentration in the water below 20 meters depth in the region between Dabob Bay and the Great Bend (PRISM Station 11) plotted versus the day of the year. The method for this calculation is explained at the end of this text. There are two important aspects of this figure that the reader should note. The first is the annual cycle in the dissolved oxygen concentrations, the second is the interannual variability and its trend in recent years.

Dissolved oxygen in the deep waters of southern Hood Canal goes through a seasonal cycle – reflecting the physical and biological processes which control its distribution. The bottom waters of Hood Canal are replaced approximately once per year around the end of summer. The “new” bottom waters are dense (salty) waters that have recently upwelled along the coast and traveled through the Strait of Juan de Fuca to Admiralty Inlet. The dissolved oxygen content of these waters are is typically higher than those within Hood Canal, but they are much lower than the concentration would be if these waters were in oxygen equilibrium with the atmosphere. This seasonal flushing results in a mid-depth dissolved oxygen minimum through Hood Canal, representing the old waters being pushed up and sandwiched between the top layer and the “new” bottom waters. The oxygen minimum can be found just below the pycnocline (density gradient just below the surface layer) over much of this region. The average dissolved oxygen in southern Hood Canal then increases during the winter and into spring. This is the result of flushing of the mid-depth waters of the canal with colder, fresher waters from outside the canal. These incoming waters are not dense enough to displace the canal’s bottom water, but have relatively a high dissolved oxygen content. Near the time of the spring phytoplankton bloom, the dissolved oxygen concentrations in the deep water begin to decrease. This is because the demand for dissolved oxygen for respiration of the sinking organic matter is greater than the supply of dissolved oxygen from waters flowing over the sill. The flushing slows greatly or ceases during the summer months until the introduction of the upwelled water again occurs in the late summer. During this late summer period the oxygen inventory is typically at its lowest.

The scatter in the average dissolved oxygen concentration during this cycle can be explained by interannual variability. Notice that this variability is greatest in the spring. An important feature, relevant to the HCDOP-IAM study, is that the majority of the data collected in the1990s and 2000s have lower concentrations than the average at a given time. The differences are small, but they seem to indicate that average dissolved oxygen concentrations have decreased over the observational record.

The measurement of oxygen is based on the same analytical technique during both timeperiods: standard oxygen titrations. The calculation of the average dissolved oxygen concentration in the volume of water that is sampled at only a few locations requires the use of sophisticated mathematical techniques. Using an optimal estimation technique (Roemmich, 1983, J. Physical Oceanography), the dissolved oxygen concentrations determined by standard titration techniques from samples collected at discrete stations and depths are fitted to a regular grid. Although the station locations and sampled depths have changed slightly between the 1950s/60s and the 1990s/2000s, the same grid has been used to calculate the average dissolved oxygen concentrations. This gridded data is also used to prepare the comparative cross-sections.