2.3 CTD and Dissolved Oxygen Sampling 



The depth gradients in temperature and salinity were 

 characterized at selected REMOTS® stations using a CTD probe 

 (Applied Microsystems, Ltd. Model STD-12®) . The STD-12® (used to 

 measure conductivity, temperature, and pressure) was mounted 

 vertically on the REMOTS® camera frame such that its sensors were 

 located approximately 42 cm from the camera base. A dissolved 

 oxygen probe (Rexnord® Instruments Model 66) , interfaced to the CTD, 

 was attached to the bottom of the REMOTS® camera frame to make 

 measurements as close to the sediment -water interface as possible 

 (approximately 6 to 9 cm above the bottom) . In this configuration, 

 vertical hydrographic profiles were obtained with each deployment 

 of the camera. 



The STD-12® is capable of sampling up to 8 scans per 

 second and can store up to 7648 scans in 56k of internal RAM (random 

 access memory) . Commands are sent to and data read from the 

 instrument with a Compaq® Portable II microcomputer via an RS-232 

 serial interface. Prior to commencing the REMOTS® survey, the 

 STD-12® was set to internally log data at 1 second intervals. The 

 data were downloaded to the microcomputer during REMOTS® film 

 changes and stored on floppy disks for later analysis. Salinity 

 was calculated from conductivity data based on procedures described 

 by Perkin and Lewis (1980) . 



The Rexnord® Model 66 probe is a polarographic oxygen 

 electrode (platinum-lead galvanic couple with a potassium-iodide 

 electrolyte) . Field calibration of the probe was performed by 

 obtaining water samples at selected REMOTS® stations approximately 

 one meter above the bottom and one meter below the surface using a 

 Niskin® bottle. Following retrieval of a water sample, the Rexnord 

 probe was placed inside the Niskin® bottle while a 300-ml subsample 

 was withdrawn for determination of the dissolved oxygen 

 concentration using a modification of the standard Winkler titration 

 method (Strickland and Parsons, 1972; Parsons et al . . 1984). A 

 calibration curve was generated by plotting the raw voltages 

 obtained from the Rexnord® probe against the Winkler DO 

 concentrations determined simultaneously in the Niskin® bottles. 



3.0 RESULTS 



3 . l Bathymetry 



Depth at the disposal point over mound "B" decreased 2 . 

 meters since the November 1987 survey, while depths at mounds "A" 

 and "C" remained unchanged (Figure 3-1) . In August 1986, mounds 

 "A", "B", and "C" had minimum depths of 29.50 m, 32.50 m, and 27.75 

 m, respectively. The November 1987 results indicated that the 

 minimum depth of mound "B" had decreased by 0.75 m to approximately 



