Project: Hypoxia Monitoring and Related Studies, Summer Shelfwide 1995

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 1994 cruise.  Hydrographic data were obtained 
from the LUMCON SeaBird CTD system, and a Hydrolab Surveyor 3.  The SeaBird data are listed in 
N795-CTD.TXT.  The Hydrolab data are reported in N795HLB.TXT.  File structure for N795-CTD.TXT 
is displayed in CTDSTRUC.TXT, and that for N795HLB.TXT is displayed in HL-STRUC.TXT.  Data for 
nutrients, pigments, and station information are provided in N795NUTA.TXT, N795PIGS.TXT, an N795-
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 4.212.  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. 4.212 modules DATCNV, ALIGNCTD and 
DERIVE.  In order to improve alignment between the oxygen values and temperature and conductivity 
values, oxygen values were advanced by 3.467 seconds using ALIGNCTD.  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.  The oxygen data for the NECOP shelfwide summer 1995 
hypoxia cruise were corrected by the follow regression: rev.D.O. = 0.843(CTD D.O.) - 0.083.  If the 
rev.D.O. value became negative, the value was changed to 0.0.  This occurred for near-bottom values at 
D2, D1N, and D1.  There was good agreement of corrected CTD values (rev.D.O.) with bottom values 
obtained from the Hydrolab CTD.

There were problems with the Hydrolab CTD conductivity measurements throughout the cruise.  Notes are 
made on the data reports, but not consistently.  Most bad values were omitted.  There are instances, 
however, where reported salinity is not consistent with measurements above and below.  These values 
should be used with caution. 

Estimated total depth was determined from the TOMMY MUNRO 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.