Project: Hypoxia Monitoring and Related Studies, Transect C, 1995

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 was conducted 
with known KCl molar solutions (specific conductance and salinity) and known pH solutions (pH).  The 
dissolved oxygen probe was calibrated to water saturated air at a known temperature and barometric 
pressure (laboratory grade barometer).  The temperature probe and depth probe were not calibrated.  The 
depth probe was field-calibrated before each deployment.  Routine maintenance was conducted on the 
Hydrolab Surveyor 3, along with an annual check by Hydrolab Corp.  Salinity samples were taken for 
laboratory determination on an AutoSal and a correction made, if necessary.  Values for salinity and % 
oxygen saturation were 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 was the 
deepest depth recorded by the Hydrolab Surveyor 3.  

Water for chlorophyll analysis (30 - 100 ml) was 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 was 
calibrated for chlorophyll a against a chemical supply house chlorophyll a standard measured on a 
spectrophotometer.  

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 were supervised and quality 
controlled by Nancy Rabalais.  Nutrient analyses were conducted by Tom Oswald under the supervision of 
R. E. Turner.  Data preparations were conducted by Ben Cole under the supervision of Nancy Rabalais.