Project: Hypoxia Monitoring and Related Studies, Transect C, 1997

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 1997 cruise.  
Hydrographic data were obtained from the LUMCON SeaBird 911+ CTD system, 
and a Hydrolab Surveyor 3.  The SeaBird data are listed in CTD-C97.TXT.  
The Hydrolab data are reported in HLAB-C97.TXT.  Data for nutrients and 
surface salinity, pigments, and station information are provided in NUSALC97.TXT, PIGSDC97.TXT, and STN-C97.TXT, respectively.  STRUC-C97.TXT provides 
information about the data files: number of records, file size, structure, 
archive date, as well as field descriptions and contacts for further details.

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 4.222  through June and thereafter with 
ver. 4.225.  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. 4.225 modules DATCNV, ALIGNCTD and DERIVE.  Two fluorometers 
were deployed on the SeaBird rosette on most cruises; a SeaTech Fluorometer 
(FLS) and a Chelsea Aquatracka Fluorometer (FLC). 

An auxiliary oxygen sensor was deployed in place of the Chelsea Fluorometer 
during the February and July cruises. The Chelsea Fluorometer was also 
out of service in during September. The SeaTech fluorometer was not in 
service during the October and November cruises. Fluorometer voltage 
values are blank when the data were unavailable.

Each month, a series of dissolved oxygen data with different advances 
were plotted and evaluated. Dissolved oxygen values were then aligned 
relative to temperature and conductivity values using the SeaSoft module 
ALIGNCTD to minimize the effect of the slower response time of the oxygen 
sensor.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 sensor end point calibrations were made prior to each cruise by the 
LUMCON CTD technician and entered into the SeaSoft configuration file. For 
all but the July cruises, the upper and lower limits were estimations; thus, 
the column headings for these dissolved oxygen values are designated approx. D.O..  Winkler titrations on board ship for the July data were 
used to develop a regression against the CTD data, which were corrected, if 
necessary.  No correction was made for the NECOP97 data. 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 for the July cruise and no correction was necessary.  The 
July data are reported as actual D.O. values and not an approximation.  If 
the SeaBird D.O. values were very different  from those recorded by the 
Hydrolab Surveyor 3, they were deleted from the data set and not reported.

Fathometer depth was determined from the PELICAN fathometer (Odum 
DF3200MkII Precision Survey Echo Sounder) value on station.

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 was made, if necessary.  Values for salinity and % oxygen saturation were derived internally by the Hydrolab Surveyor 3.  Calculations were made for 
density (sigma-t).   

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. Water for suspended sediments were taken from a flow-through system (LSU, Coastal Studies Institute).  As large a volume as possible (up to 1000 ml) was filtered through a 47-mm GF/F filter, rinsed with distilled water, then 
reweighed. 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 and suspended sediment 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.  

AutoSal samples were run by Jim Lee under the supervision of R. E. Turner.  Data preparations were conducted by Ben 
Cole under the supervision of Nancy Rabalais.N.B.  

The location of station C9 changed many times during the course 
of 1997 as we tried to find a platform to which we could secure the research 
vessel, or an unobstructed area in which to anchor.  The latitude and longitude 
are correct for C9 as listed with each set of data.

None were made to my knowledge.