



6 



5 + 

 4 



3 + 

 2 



1 + 

 

 a. 



0. 



0. 



a. 



S S S : 



a. Q. < < 



o o o c 



O C3 ^ C 

 '^ csi '^ c 



Time 



S S 5 

 < < < 



S 

 < 



S S 



< a. 



'— <NJ •— 



Figure 2-5. A 24-hour segment of the DO plot from Figure 2-4 showing a 

 lacl< of strong diurnal signal. 



ICG 

 80 

 60 

 40 

 20 



Interval 2 

 Interval 3 



4 6 



Bottom D.O. (mg/L) 



10 



Figure 2-6. Comparison of cumulative distribution functions for 

 EMAP-Virginian Province Intervals 2 and 3. 



By plotting the cumulative distribution of bottom DO concentrations as a function of their weighted frequency 

 (based on the spatial area represented by each station), we can estimate the percent area across the region 

 with a DO concentration below any given value. Figure 2-6 shows cumulative distribution functions (CDFs) 

 for polnt-in-time bottom DO measurements collected in 1990 during intervals 2 and 3 (/.e., the index period). 

 To determine the percent of estuarine area with a dissolved oxygen concentration less than 5 mg/L, one 

 would look for the point on the y axis where the curve intersects the value of 5 on the x axis (/.e., 15 to 20% 

 in Figure 2-6). Confidence intervals can also be constructed around these CDFs, but were eliminated here 

 for the sake of clarity. This figure shows that the percent area classified as hypoxic (/.e., DO <5 or <2 mg/L) 

 was approximately the same in the first half of the index period as it was in the second half. This stability 

 makes this Indicator appropriate for monitoring programs documenting the spatial extent of environmental 

 condition on large (/.e., regional) scales. 



2-11 



