to minimize the source of error. In fact, EMAP-Estuaries field crews conducted side-by-side testing during 

 training to minimize between-crew differences. Good comparability between crews was achieved. However, 

 because this was considered a training exercise, these data were not saved. Such side-by-side testing 

 could be incorporated into any future analyses of the dissolved oxygen indicator. This would need to be 

 conducted in the laboratory rather than in the field to eliminate the inherent temporal and spatial varability 

 at any given site. 



180 

 160 

 140 

 120 



C 100 

 o 



3 



g" 80 



60 



40 



20 







Total N = 784 



JX^ 



* I ^1 I 



-+- 



-I- 



-(- 



1 02 0.3 04 05 06 OJ 08 09 1 11 1.2 13 14 15 16 1.7 1 



A D.O. (mg/L) 



1.9 2 2 1 22 23 24 25 26 27 28 29 3 



Figure 2-3. Frequency distribution of EMAP dissolved oxygen quality assurance data. A DO 

 represents the absolute difference between the CTD measurement and that 

 from a second instrument. Over 90% of the stations met the measurement quality 



objective (A DO < 0.5 mg/L) . 



Other potential sources of measurement error include inadequate thermal equilibration of the instrumentation 

 prior to conducting a cast, and allowing insufficient time for the DO probe to repond to changes in DO 

 concentration across an oxycline. Both can be addressed by proper training and evaluated by examining 

 the full vertical profile for several parameters {i.e., temperature and DO). 



Guideline 9: Temporal Variability • Within the Field Season 



it is unlikely in a monitoring program that data can be collected simultaneously from a large number of 

 sites. Instead, sampling may require several days, weeks, or months to complete, even though the 

 data are ultimately to be consolidated into a single reporting period. Thus, within-field season variability 

 should be estimated and evaluated. For some monitoring programs, indicators are applied only within 

 a particular season, time of day, or other window of opportunity when their signals are determined to be 

 strong, stable, and reliable, or when stressor influences are expected to be greatest. This optimal time 

 frame, or index period, reduces temporal variability considered irrelevant to program objectives. The 

 use of an index period should be defended and the variability within the index period should be estimated 

 and evaluated. 



2-9 



