46 
With some clear exceptions, the day-night concentration differences in these buoy 
data are small. Back River (segment BACOH), a tidal river known to be stressed by 
discharges from a large urban sewage treatment facility, exhibits the largest day- 
night difference in mean and median concentrations: -2.24 mg liter' 1 and -4.51 mg 
liter 1 , respectively (Table V-4). Note that here the nighttime concentration is higher 
than during the daytime, which seems counterintuitive. But, in fact, the average 
day/night difference in the daily means and medians is almost always negative in this 
table. A buoy site in the lower Potomac River (POTMH) and one in upper Potomac 
River (POTTF) showed day-night differences greater than 1 mg liter 1 in the daily 
mean or median or both, but all other sites showed differences less than 1 mg liter -1 . 
The average day-night differences in the daily minimum concentration and lowest 
1 percent value were similarly generally small, but with more sites exhibiting day- 
night differences in excess of 1 mg liter -1 : mesohaline Patapsco River (PATMH), 
tidal fresh (POTTF) and mesohaline (POTMH) Potomac River, tidal fresh James 
River (JAMTF), middle central and lower western mainstem Chesapeake Bay 
segments CB4MH and CB6PH, respectively, and Tangier Sound (TANMH). In 
contrast to the findings for the daily mean and median, the concentration minima and 
lowest 1 percent were generally higher in the daytime than at night. 
30-Day Mean and Instantaneous Minimum Criteria Attainment 
Table V-5 shows how the continuous dissolved oxygen measurements stack up 
against the corresponding designated use dissolved oxygen criteria. The dissolved 
oxygen criteria are to be assessed for each segment/designated use separately. Thus, 
in this analysis, the day and night measurements are pooled and the mean, 1 percent 
concentration and other statistics are calculated within month, if the data record 
extends over multiple months. Asterisks flag the continuous buoy data records where 
the 30-day mean criterion is not achieved (i.e., monthly mean dissolved oxygen 
concentration is lower than the applicable criterion) or where the measured 1 percent 
dissolved oxygen concentration is lower than the instantaneous minimum criterion. 
Looking down the columns in Table V-5 labeled “30-day Mean” and “Instantaneous 
Minimum” under the heading “Criterion Not Achieved”, it can be seen frequently 
that if the 30-day mean criterion was achieved, the instantaneous minimum criterion 
was also achieved. Conversely, if the 30-day mean criterion was not achieved, the 
instantaneous minimum criterion also was not achieved. Further, if only one 
dissolved oxygen criterion was not achieved, then it was usually the instantaneous 
minimum criterion that was not achieved. 
Table V-6 summarizes the criteria achieved/not achieved rate by segment and desig¬ 
nated use and Table V-7 pools the Table V-6 findings by designated use. For the 
open-water designated use, in 80 out of 94 cases (—85 percent), if the 30-day mean 
criterion was achieved/not achieved, then the same was the case for the instantaneous 
minimum criterion. In deep-water designated use habitats, this condition was true in 
15 out of 26 cases (~57 percent). The diversity of buoys deployed in deep-channel 
designated use habitats is too small for drawing very specific conclusions at this time. 
chapter v • Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
