8. Dissolved Oxygen as a Water Quality Response Measure 
Dissolved oxygen (DO) is an important water quality metric because of its effects on the well¬ 
being of estuarine resident and transitory organisms. Salmon and trout are particularly esteemed fishes 
in the PNW and traverse the estuaries during upstream and downstream migrations. The dissolved 
oxygen criterion for Oregon’s estuaries and streams focuses on the oxygen concentration needed for 
these fish because of their socioeconomic importance and their requirement for comparatively high 
oxygen levels. As a result, “salmon and trout rearing and migration” is a common designated use for 
Oregon coastal estuaries, including the Yaquina Estuary and River. 
Two species of salmon that are of particular importance in the Yaquina are the coho salmon 
(Onchorhynchus kisutch ) and steelhead trout ( O. mykiss). The reduced size of Oregon coastal coho 
populations have been a cause of particular concern. Chinook salmon ( O. tshawytscha) spawn and 
occur as juveniles in brackish waters, and are also present in the Yaquina River. The cutthroat trout 
( O. clarki ) is a fourth important salmonid species found in the Yaquina system, and a portion of this 
population also follows the salmon life history of migrating to the sea, where it grows to adulthood 
before returning to the natal stream to spawn. The timing of salmonid migrations through the estuaries 
varies by species, and is influenced by local conditions and hydrology. However, adults generally 
enter the estuary in the fall and progress upstream to freshwater spawning streams. The juvenile 
outmigrants, termed “smolts,” typically move downstream during the months of March to June. 
8.1 Seasonal, Zonal and Long-term Trends in Dissolved Oxygen 
There are strong seasonal patterns in dissolved oxygen within the Yaquina Estuary (Figure 8.1). 
Oxygen levels (expressed as both mg l" 1 and % saturation) in the estuary are comparatively stable 
during the wet season, but show a decline during the dry season. The wet season dissolved oxygen 
have an overall mean value of 9.7 mg f 1 (n = 869) dissolved oxygen. The dry season data were fitted 
using a nonlinear least squares procedure to have a descending cosine curve that begins at the wet 
season value of 9.7 mg l' 1 , declines to a value of 5.8 mg f 1 on August 2, and then returns to the wet 
season value. Zones 1 and 2 appear to follow the same pattern. Subsequent data analyses used the 
deviations from this modeled seasonal pattern (the solid line in Figure 8.1), so that the overall seasonal 
changes in oxygen concentration and differences in sampling would not confound more detailed 
analyses. All dissolved oxygen values used in the analyses were collected during daylight hours. 
Therefore, diel cycling of oxygen values due to plant photosynthesis and respiration are not 
< i- 
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