standard against which individual data points are assessed. The individual data are 
thus scored between 1 and 100. The assessments are conducted separately in salinity 
classification and in depth layers corresponding to the designated uses. The median 
score of the individual data scores is then calculated. The benchmark distribution is 
divided roughly into thirds, which are defined as ‘good’, ‘fair’ and ‘poor’ (these 
terms relate only to each other, not necessarily to actual water quality requirements 
of living resources). Status of the parameter is assigned depending on where the 
median score falls among these divisions. 
Using this procedure, open-water concentrations of the four parameters were 
assessed for each Chesapeake Bay Program segment, yielding for each parameter an 
assessment of ‘good,’ ‘fair’ or ‘poor’ for each segment, year and season (spring and 
summer). To qualify as a reference location, at least three out of four parameters had 
to be ‘good’ and only one parameter could be ‘fair’. Once the times and locations 
were selected, the corresponding monthly average dissolved oxygen concentration 
data were evaluated against the migratory fish spawning and nursery dissolved 
oxygen criterion value of 6 mg liter' 1 (evaluated as a 30-day mean, not as a 7-day 
mean) and the open-water dissolved oxygen 30-day mean criterion of 5 mg liter' 1 for 
spring and summer, respectively. The percent volume failing the criterion was calcu¬ 
lated for each month of the season/year. The resulting cumulative frequency 
distribution curves are shown in figures VI-15 and VI-16, respectively. Figure VI-16 
currently serves as the recommended reference curve for both the migratory fish 
spawning and nursery and open-water fish and shellfish designated uses for purposes 
of assessing dissolved oxygen criteria attainment. 
Deep-Water Dissolved Oxygen Criteria Reference Curve 
Reference areas were identified using a measure of benthic community health, the 
Chesapeake Bay Benthic Index of Biological Integrity (benthic-IBI; Weisberg et al. 
1997). Sessile benthic communities are good indicators of water quality conditions of 
overlying waters. Although relatively tolerant of lower oxygen concentrations, a 
dissolved oxygen concentration of 2 mg liter' 1 is considered the lower threshold below 
which benthic infaunal communities become severely stressed (see Chapter III). A 
healthy benthic community, therefore, could indicate that dissolved oxygen conditions 
meeting deep-water dissolved oxygen criteria were met. Benthic infaunal community 
samples are collected as part of a long-term Chesapeake Bay Benthic Monitoring 
Program. Samples are collected at fixed and random locations in the summer season, 
usually in August/September. If the benthic-IBI of that sample is ‘good’, in this case 3 
or greater on a scale of 1 to 5, then it is likely that dissolved oxygen conditions have 
been adequate for the previous one to two months of the summer. 
The benthic-IBI data from 1985 through 1994 were assessed and a list of deep-water 
reference locations identified by year and segment was compiled. Then, the summer 
(June through September) dissolved oxygen data that were collected as part of the 
Chesapeake Bay Water Quality Monitoring Program at the times and places on the 
list were evaluated relative to the deep-water criteria. Figure VI-17 shows the 
chapter vi 
Recommended Implementation Procedures 
