Project: Hypoxia Monitoring and Related Studies, Summer Shelfwide 1996

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

Two sets of CTD data were taken for the summer shelfwide 1996 cruise.  Hydrographic 
data were obtained from the LUMCON SeaBird CTD system, and a Hydrolab Surveyor 3.  
The SeaBird data are listed in N796-CTD.TXT.  The Hydrolab data are reported in 
N796HLAB.TXT.  File structure for N796-CTD.TXT is displayed in CTDSTRUC.TXT, 
and that for N796HLB.TXT is displayed in HLSTRUC.TXT.  Data for nutrients and 
surface salinity, pigments, and station information are provided in N796NUTA.TXT, 
N796PIGS.TXT, an N796-STN.TXT, respectively.  The structure files for these are 
NUTSTRUC.TXT, PIGSRUC.TXT, and STNSTRUC.TXT. 

For the SeaBird CTD, temperature, pH, dissolved oxygen, conductivity, salinity, 
fluorescence, % light transmission, and depth were measured with p-sensor depths 
reported as measured.  Data were archived with SeaBird software SeaSoft version 4.217.  
Data reductions from the SeaBird were generated by Ben Cole and quality controlled by 
Nancy Rabalais.  All scans were processed without averaging or interpolation with a bin 
size of one scan, using SeaSoft vers. 4.217 modules DATCNV, ALIGNCTD and 
DERIVE.  

In the early part of the cruise (stations A'1, A'2, A'3 and A7), the pump was connected to 
D.O. sensor 1.  There were no CTD data for Stations A'4 and A'5.  Beginning with station 
A6 and continuing throughout the cruise, sensor 1 was removed from the pumped circuit 
and replaced with sensor 2.  The CTD plumbing configuration did not change.  Sensor 1 
was removed from the pumped circuit and replaced with sensor 2.  The data file for station 
B4 was damaged and unusable.  Oxygen data for stations A'1 and A7 were deleted because 
the values were unrealistic.  Oxygen data for stations A'2 and A'3 were left in the data base, 
but noted as "suspect."

In order to improve alignment between the oxygen values and temperature and 
conductivity values, oxygen values were advanced by 4.5 seconds using ALIGNCTD.  A 
special protocol of equal lowering and raising rates was observed at 10 stations.  An 
optimal advance was derived for each station.  Results ranged from 4.0 to 4.9 seconds.  
The average value was 4.5 seconds and was applied to all CTD casts.  A small portion of 
the delay in the D.O. sensor response is attributable to the age of the sensor membrane.  
Both sensors had been recently reconditioned by SeaBird.  The other part of the delay is 
caused by the transit time of a parcel of water through the pump-tubing-plenum plumbing 
circuit.

Data reported were derived from downcasts.  Downcast scans selected for each CTD 
station were chosen to illustrate: 1) data values as near to the surface as possible, 2) data 
values at whole meter increments, and 3) data values as close to the bottom as the CTD 
was lowered.  In certain cases where data values of a parameter changed significantly 
between whole meter increments, 0.1 or 0.2 meter scans were selected.

The SeaBird CTD was factory calibrated for salinity and temperature.  Oxygen 
concentration determinations, upper and lower end, were made prior to the cruise by the 
LUMCON CTD technician and entered into the SeaSoft configuration file.   Winkler 
titrations on board ship were used to develop a regression against the CTD data, which 
were corrected, if necessary.  Winkler titrations were conducted under the supervision of 
Nancy Rabalais.  There was good agreement of CTD values with Winkler values and 
bottom values obtained from the Hydrolab CTD.

Fathometer depth was determined from the PELICAN fathometer value on station.

Water for chlorophyll, nutrient, and salinity analyses were collected from the surface by 
bucket, from mid-water in 5-l Niskin bottles on the CTD/rosette system, and from bottom 
in a 5-l bottom tripping Niskin bottle secured horizontally in a frame with the Hydrolab 
Surveyor 3, or a 5-l Niskin on the CTD/rosette system.  Depth of 0.00 indicates a bucket 
sample collected from the surface of the water.  Deepest depths of water samples were 
from the bottom-tripping Niskin and correspond to the deepest depth recorded from the 
Hydrolab.  Other depths indicate the p-sensor reading for 5-l Niskin bottles on the rosette.  
p-sensor was located approx. 1 meter below the mid-point of the 5-l Niskin.  

Water for chlorophyll analysis (30 - 100 ml) was filtered on board ship through GF/F 
filters, which were then fixed in 5 ml of DMSO/90% acetone (40/60) solution, allowed to 
extract for at least one hour 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).  Salinity determinations for surface water samples 
were made on an AutoSal maintained by the LSU Coastal Studies Institute Shop.  Pigment 
measurements were supervised and quality controlled by Nancy Rabalais.  Nutrient 
analyses were conducted by Tom Oswald under the supervision of R. E. Turner.