28 
River, whose hypoxic/anoxic conditions are of long standing and whose dynamics 
are well enough understood to be modeled mathematically and relatively precisely. 
There are other segments that have long term monthly and twice monthly dissolved 
oxygen concentration records whose station coverage is considered to represent the 
whole segment adequately or at least areas most likely to have dissolved oxygen 
concentrations below saturation levels. The Chesapeake Bay Program partners have 
previously demonstrated (see Chesapeake Bay Dissolved Oxygen Goal for Restora¬ 
tion of Living Resource Habitats ; Jordan et al. 1992) that relatively good predictive 
models can be developed for segments that suffer hypoxia at some regular frequency 
and so far have demonstrated no long term trend in dissolved oxygen concentrations. 
These models produce estimates of the percent of time the segment depth is below 
some specified concentration. These monitoring data-based models reflect only 
daytime measurements, but can be enhanced (and validated) by the in-situ contin¬ 
uous records from the buoy deployments. 
The remaining segments not characterized above are those segments where the long¬ 
term fixed monitoring stations, sampled on a monthly to twice-monthly basis, do not 
well represent dissolved oxygen conditions elsewhere in the segment. Typically 
these segments have a moderately deep channel with flanking nearshore areas of 
significant size. In these segments, tidal pulses from downstream, inflows from 
upstream, and local land-based influences vary in their dominance, and the current 
long-term water quality monitoring data do not capture ephemeral events or the near¬ 
shore conditions very well. The new shallow water monitoring component of the 
larger Chesapeake Bay Water Quality Monitoring Program is designed to generate 
the additional data necessary to assess criteria attainment in these segments. The 
Chesapeake Bay Program partners are now accumulating such data for a growing 
number of Chesapeake Bay Program segments. 
ASSESSMENT OF INSTANTANEOUS MINIMUM 
CRITERIA ATTAINMENT FROM MONTHLY MEAN DATA 
By overlaying information from the buoy data about diurnal variability and the 
frequency of common hypoxic events, such as those caused by phytoplankton bloom 
respiration and decay, pycnocline tilting, etc., on top of the long-term fixed-station 
monitoring data record, we can better understand the relationship between attain- 
ment/non-attainment of the 30-day mean and instantaneous minimum criteria. The 
reader should keep several things in mind. The temporal record of the long-term, 
fixed-station monitoring program is considered “low-frequency” relative to the high 
frequency record of the “continuous” data record from the buoy deployments. The 
available continuous records chronicle a few days to months of a single year. Each 
measurement is closely related to the previous and next measurement, providing a 
detailed record of the dissolved oxygen response to the specific conditions of that 
period. These buoy data records are measuring conditions at a single fixed point in 
the water column, usually about a meter off the bottom in these data sets. The sensors 
chapter v 
Guidance for Attainment Assessment of Instantaneous Minimum and 7-Day Mean Dissolved Oxygen Criteria 
