The dissolved oxygen concentration of estuarine water is highly dependent on a variety of factors, including 

 photosynthesis (which is affected by nutrient levels), temperature, salinity, tidal currents, stratification, winds, 

 and water depth. These factors make DO concentrations highly variable over relatively short time periods. 

 There is also a strong seasonal component, with lowest dissolved oxygen concentrations experienced 

 during the summer months of late July through September. In the EMAP program, estuarine monitoring 

 was conducted during the summer when the biotic community is most active. Since we are interested in 

 DO because of its effects on aquatic biota, and since summer is the season when organisms are most 

 active and dissolved oxygen concentrations are generally the lowest, it is also the most appropriate season 

 for evaluating the extent of hypoxic conditions. In 1 990, EMAP conducted sampling in the Virginian Province 

 estuaries to determine the most appropriate index period within the summer season. A subset of stations 

 were sampled ineachofthreesamplingintervals;20 Juneto18 July, 19 July to 31 August, and 1 September 

 to 22 September. The results of analysis of the data collected at these stations showed the DO concentrations 

 to be most consistent in Intervals 2 and 3, suggesting that July 19-September 22 is the most appropriate 

 definition of the index period for the study area. Similar reconnaissance would need to be performed in 

 other parts of the country where this indicator may be employed. 



Even within the index period, DO concentrations at a given station vary hourly, daily and weekly. The high 

 degree of temporal variability in DO at one station over the period from July 28 through August 26 is shown 

 in Figure 2-4. 



Figure 2-4. Continuous plot of bottom dissolved oxygen concentration at EMAP 

 station 088 in Chesapeake Bay, 1990. 



Figure 2-5 illustrates a 24-hour record of bottom dissolved oxygen from the same station. Although 

 concentrations vary throughout the day, most mid-Atlantic estuaries generally do not exhibit a strong diurnal 

 signal; most of the daily variability is associated with other factors such as tides (Weisberg et al. 1 993). This 

 is not the case in other regions, such as the Gulf of Mexico, where EMAP showed a strong diurnal signal 

 (Summers et al. 1993). Such regional differences in temporal variability illustrate the need to tailor 

 implementation of the indicator to the specific study area. 



Short-term variability, as illustrated in Figures 2-4 and 2-5, makes this indicator, using single point-in-time 

 measurements, inappropriate for characterizing a specific station. However, single stations are not the 

 focus of the program for which this indicator is being evaluated in this example. The purpose of EMAP is to 

 evaluate ecological condition across a broad geographic expanse, not at individual stations. The percent 

 area hypoxic throughout the index period is more stable on a regional scale. 



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