8.2 Percentile Approach for Dissolved Oxygen 
Dry season DO levels in the Yaquina Estuary are comparable to those found in other Oregon 
estuaries (Table 8.1) and are relatively high compared to other estuaries in the U.S. (U.S. EPA, 2004a). 
There was not a significant difference in dissolved oxygen levels in Zone 1 of the Yaquina Estuary 
(dry season using discrete samples) and Zone 1 of the other estuaries sampled in the Classification and 
NCA data sets (Kruskal-Wallis one way ANOVA on ranks, p> 0.05). The dissolved oxygen levels in 
Zone 2 of the Yaquina Estuary (dry season using discrete samples) are significantly lower than those in 
Zone 2 of the estuaries sampled in the Classification and NCA (including Columbia) datasets (Dunn's 
Method for pairwise comparison, p< 0.05). In the Yaquina Estuary during the dry season, the DO 
levels do not meet the Oregon criterion of 6.5 mg f 1 for 25% and 19% of the time in Zones 1 and 2, 
respectively (using discrete samples). There was not a statistically significant difference in dry season 
median DO levels between Zones 1 and 2 (calculated using recent discrete data; Mann-Whitney Rank 
Sum, p>0.05). During the wet season, DO conditions do not appear to be a cause for concern. 
There is considerable temporal variability in DO levels, which is not well captured in discrete 
point measurements. Continuous data are valuable in that they provide insight into the processes 
influencing observations such as the import of hypoxic water (Section 8.1), and they can allow 
evaluation of DO levels during both day and night conditions. Continuous data were available from 
datasondes deployed at two locations (Zone 1-3.7 km from the estuary mouth, Zone 2-18 km from 
the estuary mouth) in the estuary. The data were inspected to ensure that biofouling was not 
influencing observations, and only data from the first 7 days of each deployment was included in the 
analysis. In Table 8.1 we present the percentiles of the discrete and continuous data for the dry season 
for comparison; however, we did not perform formal statistical analyses due to the large difference in 
sample size. Median DO levels are lower for the continuous data compared to the discrete data, 
particularly in Zone 1 
Using the continuous data, we examined how often the State of Oregon DO criterion was not 
met during May-October of 2006. Dissolved oxygen levels fell below the 6.5 mg f 1 criterion 37% and 
28% of the time in Zones 1 and 2, respectively. The frequencies that observations fall below the 
criterion in the two zones are comparable but slightly higher than those calculated from the discrete 
samples (Table 8.1). A plot of salinity versus DO for the 2006 datasonde data has a pattern of low DO 
at high salinities (similar to that presented in Figure 8.3b) for the station in Zone 1; however, this 
pattern is absent in the datasonde data from the Zone 2 station. Based on these patterns, the below 
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