Guideline 10: Temporal Variability - Across Years 



Indicator responses may change over time, even v\/hen ecological condition remains relatively stable. 

 Observed changes in this case may be attributable to v\/eather, succession, population cycles or other 

 natural inter-annual variations. Estimates of variability across years should be examined to ensure that 

 the indicator reflects true trends in ecological condition for characteristics that are relevant to the 

 assessment question. To determine inter-annual stability of an indicator, monitoring must proceed for 

 several years at sites known to have remained in the same ecological condition. 



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Dissolved Oxygon (mg/L) 



Figure 2-7. Annual cumulative distribution functions of bottom dissolved 

 oxygen concentration for the Virginian Province, 1990-1993. 



As discussed above, point-in-tlme DO measurements can be highly variable at any given station. This 

 applies to across-year comparisons as well as within-season comparisons. However, when using this 

 information to address the spatial extent of hypoxia across a broad region, this indicator is reasonably 

 stable. Figure 2-7 shows the similarity of individual CDFs of bottom dissolved oxygen concentration in the 

 estuaries of the Virginian Province for 1990 through 1993. Figure 2-8 shows the percent area below 5 and 

 2 mg/L (defined by EMAP as criteria for hypoxic and very hypoxic conditions, respectively) for those same 

 years. Note that the percent area considered hypoxic by these criteria do not differ significantly from year to 

 year, despite differences in climatic conditions (temperature and rainfall: Figure 2-9). 



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