instantaneous minimum criteria should not be assessed unless data are available for 
a specific location/segment at a temporal scale consistent with the instantaneous 
minimum duration. 
Spectral Analysis Approach. The foundation for this method was developed by 
Neerchal et al. (1992) in the context of implementing the Chesapeake Bay dissolved 
oxygen restoration goal (Jordan et al. 1992) and has been modified for criteria appli¬ 
cation. The method uses spectral analysis to extract the cyclical components of the 
long- and short-term time-series records and combines them to create a synthesized 
time-series data set with data synthesized at user-specified time steps. At present, the 
synthetic data are hourly, with cyclic components limited to two cycles per day. The 
synthetic data have the annual and seasonal cyclic and trend characteristics of the 
long-term record as well as the tidal, diurnal and any other periodic characteristics 
of the short-term, high-frequency record. The long-term record comes from fixed- 
station monitoring data collected at regular once or twice monthly intervals in the 
seasons of interest. The short-term data come from in-situ semicontinuous oxygen 
monitors deployed on buoys or other fixed structures at designated locations around 
the Chesapeake Bay and its tidal tributaries. These semicontinuous oxygen monitors 
are put in place for various lengths of time at many different locations and depths. 
Sites are chosen in order to best characterize the dissolved oxygen conditions in each 
designated use. The sampling interval of the semicontinuous monitors are commonly 
5, 10 or 20 minutes. To be most useful, the interval should be no longer than one 
hour. More details are provided in Appendix I. 
Application of the Spectral Analysis Approach. The spectral analysis application 
shown in Figure VI-23 uses long-term data from station CB4.2C, a monitoring 
station in the midregion of the Chesapeake Bay, and a two-month series of contin¬ 
uous dissolved oxygen measurements at a buoy deployment near that station at 
approximately 9 meters below the surface. Figure VI-23 shows the observed monthly 
dissolved oxygen concentrations (asterisks) at station CB4.2C (8- to 10-meter depth) 
and the long-term forecast (line) from the spectral equation. 
Figure VI-23. Observed monthly dissolved oxygen concentrations (*) at Chesapeake Bay 
Monitoring Program station CB4.2C (at the 8 to 10 meter depth) from January 1985 to 
January 2000 and the long-term 'forecast' (—) from application of the spectral equation. 
chapter vi • Recommended Implementation Procedures 
