3.2.3 Water-Column Stratification 
As an indicator of water-column stratification, an index of the variation 
between surface and bottom water densities was calculated from temperature 
and salinity data. The index (Ao t ) is the difference between the computed bottom 
and surface o t values, where o t is the density of a parcel of water with a given 
salinity and temperature relative to atmospheric pressure. 
The Ao t index for the 140 stations from waters of the West Coast shelf for 
which data were available ranged from 0.9 to 10.6. Approximately 30.5% of the 
area of waters of the West Coast shelf had Ao t index values greater than 2, 
indicating strong vertical stratification of the water column. The mean 
stratification index was greatest for waters off Washington and least for California 
waters (Figure 3.2.6). The mean stratification index was lowest for the CA NMS 
locations and less than half the mean for the CA non-NMS stations. During the 
sampling of the central California coast where three of the CA NMS are located, 
extremely high winds were encountered, and it is likely that wind induced 
upwelling greatly reduced water-column stratification in this region. The Bakun 
upwelling index reflects the intensity of large-scale, wind-induced coastal 
upwelling based on estimates of offshore Ekman transport driven by geostrophic 
wind stress. Index values for 36° N latitude for the West Coast in June 2003 
(source: 
http://www.pfeg.noaa.gov/products/PFEL/modeled/indices/upwelling/NA/upwell_ 
menu_NA.html) showed that the peak upwelling period for the month occurred in 
the period June 17-24, exactly at the time when the CA NMS stations were being 
sampled (Figure 3.2.7). 
3.2.4 Dissolved Oxygen 
The range of dissolved oxygen (DO) concentrations in the surface waters 
of the West Coast shelf (data available for 140 stations) was 4.1 mg/L to 13.3 
mg/L. U.S. EPA (2000a) proposed that a DO value below 2.3 mg/L is harmful to 
the survival and growth of marine animals based on data from the Virginian 
biogeographic province. A DO value of > 4.8 mg/L is considered the chronic 
protective value for growth, i.e. the ceiling above which DO conditions should 
support both survival and growth of most marine species. Values between 2.3 
and 4.8 mg/L are potentially harmful to larval recruitment, depending on duration. 
Only approximately 2.6 % of the area of waters of the West Coast shelf had 
surface DO concentrations < 4.8 mg/L. The 50 th percentile of area had a 
surface-water DO concentration of 9.8 mg/L. Surface DO concentrations were 
higher in Washington and Oregon waters than in California and higher in the OC 
NMS than in the CA NMSs (Figure 3.2.8). 
Bottom-water DO concentrations region-wide ranged from 2.1 to 8.3 mg/L 
across the 140 stations with acceptable DO data. Unfortunately, an instrument 
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