Project: Hypoxia Monitoring and Carbon Process Studies, Transect C, 1994

PIs: Nancy N. Rabalais, LUMCON; R. Eugene Turner, LSU; William J. 
Wiseman, Jr., LSU

Temperature, pH, dissolved oxygen, conductivity, salinity and depth were 
measured with a Hydrolab Surveyor 3 multiparameter water quality logging 
system.  Calibration pre- and post-cruise is conducted with known KCl 
molar solutions (specific conductance and salinity) and known pH 
solutions (pH).  The dissolved oxygen probe is calibrated to water 
saturated air at a known temperature and barometric pressure (laboratory 
grade barometer).  The temperature probe and depth probe are not 
calibrated.  The depth probe is field-calibrated before each deployment.  
Routine maintenance is conducted on the Hydrolab Surveyor 3, along with 
an annual check by Hydrolab Corp.  Salinity samples are taken for 
laboratory determination on an AutoSal and a correction made, if 
necessary.  Values for salinity and % oxygen saturation are derived 
internally by the Hydrolab Surveyor 3.  Calculations were made for 
density (sigma-t).  

Data were omitted from the top of the cast if concentrations indicated 
that the CTD probes had been brought up into the air and not 
equilibrated once back in the water.   

Water for chlorophyll and nutrient analyses were collected from the 
surface by bucket and from mid-water in 5-l Niskin bottles on a cable.  
Discrete depth samples at C6B were determined by a combined Hydrolab-
Niskin bottle.  Near-bottom water samples were collected from 5-l 
Niskins bottles on a cable taken to as close to the bottom as possible.  
The corresponding depth for the near-bottom water sample is the deepest 
depth recorded by the Hydrolab Surveyor 3.  

Water for chlorophyll analysis (40 - 100 ml) is filtered on board ship 
through GF/F filters, which are then fixed in 5 ml of DMSO/90% acetone 
(40/60) solution, allowed to extract for at least two hours in the dark, 
then measured pre- and post-acidification on a Turner Model 10 
fluorometer.  The fluorometer is calibrated for chlorophyll a against a 
chemical supply house chlorophyll a standard measured on a 
spectrophotometer.  Occasionally, water samples for chlorophyll are 
filtered, frozen in liquid nitrogen, then returned to the laboratory for 
subsequent analyses.

Samples for nutrients were frozen on board ship for later analysis in 
the laboratory.  Nitrogen and phosphorus were analyzed according to 
methods described in EPA publication EPA 600/4-79-020 (1979), method 
350.1 for ammonia-N, method 353.2 for nitrate/nitrite-N, and method 
365.1 for phosphate-P.  Silicate was analyzed according to Technicon 
Industrial method 186-72 W/B (1977).  

Hydrolab data, chlorophyll determinations, and general cruise logistics 
are supervised and quality controlled by Nancy Rabalais.  Nutrient 
analyses are conducted by Tom Oswald under the supervision of R. E. 
Turner.  Data preparations are conducted by Ben Cole under the 
supervision of Nancy Rabalais.