The third aspect to be considered is where in the water column to mal<e the measurements. Because 

 hypoxia is generally most severe near the bottom, a bottom measurement is critical. For this program, we 

 will be considering a vertical profile using a CTD. This provides us with information on the DO concentration 

 not only at the bottom, but throughout the water column. The additional information can be used to deter- 

 mine the depth of the pycnocline (a sharp, vertical density gradient in the water column), and potentially the 

 volume of hypoxic water. Using a CTD instead of a DO meter provides ancillary information on the water 

 column (salinity, temperature, and depth of the measurements). This information is needed to characterize 

 the water column at the station, so using a CTD eliminates the need for multiple measurement with different 

 instruments. 



The proposed methodology consists of lowering a CTD through the water column to obtain a vertical profile. 

 The instrument is connected to a surface display. Descent is halted at one meter intervals and the CTD held 

 at that depth until the DO reading stabilizes. This process is continued until the unit is one meter above the 

 bottom, which defines the depth of the bottom measurement. 



Guideline 4: Logistics 



The logistical requirements of an indicator can be costly and time-consuming. These requirements 

 must be evaluated to ensure the practicality of indicator implementation, and to plan for personnel, 

 equipment, training, and other needs. A logistics plan should be prepared that identifies requirements, 

 as appropriate, for field personnel and vehicles, training, travel, sampling instruments, sample transport, 

 analytical equipment, and laboratory facilities and personnel. The length of time required to collect, 

 analyze and report the data should be estimated and compared with the needs of the program. 



The collection of dissolved oxygen data in the manner described under Guideline 3 requires little additional 

 planning over and above that required to mount a field effort involving sampling from boats. Collecting DO 

 data adds approximately 1 5 to 30 minutes at each station, depending on water depth and any problems that 

 may be encountered. The required gear is easily obtainable from a number of vendors (see Guideline 7 for 

 estimated costs), and is compact, requiring little storage space on the boat. Each field crew should be 

 provided with at least two CTD units, a primary unit and a backup unit. Operation of the equipment is fairly 

 simple, but at least one day of training and practice is recommended before personnel are allowed to collect 

 actual data. 



Dissolved oxygen probes require frequent maintenance, including changing membranes. This should be 

 conducted at least weekly, depending on the intensity of usage. This process needs to be worked into 

 logistics as the membrane must be allowed to "relax" for at least 12 hours after installation before the unit 

 can be recalibrated. In addition, the dissolved oxygen probe must be air-calibrated at least once per day. 

 This process takes about 30 minutes and can be easily conducted prior to sampling while the boat is being 

 readied for the day. 



No laboratory analysis of samples is required for this indicator; however, the data collected by field crews 

 should be examined by qualified personnel. 



In summary, with the proper instrumentation and training, field personnel can collect data supporting this 

 indicator with only minimal effort. 



2-5 



