Project: Hypoxia Monitoring and Related Studies, Summer Shelfwide 1994

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 N794-CTD.TXT.  
The Hydrolab data are reported in N794HLB.TXT.  File structure for N794-
CTD.TXT is displayed in CTDSTRUC.TXT, and that for N794HLB.TXT is 
displayed in HL-STRUC.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 4.202a.  Data reductions from the 
SeaBird were generated by Ben Cole and quality controlled by Nancy 
Rabalais.  All scans of data were processed using SeaSoft ver. 4.201 
without averaging or interpolation by selecting the options "scan number 
bins" with a "bin size of 1 scan" and "include all scans (do not use the 
passive filter)."  In order to improve alignment between the oxygen 
sensor values and other CTD sensor values, the SeaSoft module ALIGNCTD 
was used to determine the optimum advance of dissolved oxygen values 
relative to temperature and conductivity values: +4.7 seconds.  When CTD 
data were processed, dissolved oxygen values were advanced +4.7 Seconds.  
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 1994 
hypoxia cruise were not corrected; CTD oxygen values and Winklers gave a 
strong, good relationship.  

Problems began with SeaBird CTD data at Station D'6.  SeaBird CTD data 
for Stations F1 and F2 were deleted because of suspect oxygen and sigma-
t values.  No SeaBird CTD data were obtained for Stations D'6, D1N, E5, 
F6, F7, F5, F4, F1, G3, G4, G5, and all stations on Transects H, I, J, 
and K. 

Estimated total depth was determined from the PELICAN JVF fishfinder 
value on station, with addition of a constant for distance from the 
transducer to the water surface.

Hydrolab cast time is from the Hydrolab; CTD cast time is the same as 
the Hydrolab, but actually occurred approx. 15 minutes before the 
Hydrolab cast time.

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 30-l bottom tripping Niskin 
bottle secured 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.  Bottom 
5-l Niskins on rosettee were used to collect "bottom" water samples for 
Stations A7, A6, B1, B2, B6, B8, C7, C6B, C4, D1, E2, E2A, AND F3.  

Water for chlorophyll analysis (40 - 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.