Project: Hypoxia Monitoring and Related Studies, Summer Shelfwide 1997

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

Two sets of CTD data were taken during 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-N97.TXT.  The Hydrolab data are reported in HLAB-N97.TXT.  Data for nutrients and surface salinity, pigments, suspended sediments and station information are provided in NUSALN97.TXT, PIGSDN97.TXT, and STN-N97.TXT, respectively.  STRUCN97.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 and processed using SeaBird software SeaSoft version 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.  

In order to improve alignment between the oxygen values and temperature and conductivity values, oxygen values were advanced by 6.47 seconds using ALIGNCTD.  A special protocol of equal lowering and raising rates was observed at 10 stations.  An optimal advance was derived for each special protocol station.  Results ranged from 5.5 to 7.5 seconds.  The average value was 6.47 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 sensor end point calibrations 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.  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.

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.

Percent oxygen saturation for Hydrolab values above 175% saturation were derived from the figure given in Green and Carritt, 1967, J. Mar. Res., 25(2):140-147.

N.B. There may have been a problem with samples collected with the bottom tripping Niskin. Surface AutoSal samples were collected at each station in order to get precise salinity values for the bucket samples. On transect C in addition to surface salinity samples, bottom salinity samples were collected from the Niskin. The salinity samples were read after the cruise in the laboratory using both the HydroLab and the AutoSal.

AutoSal salinity readings matched those taken in the lab with the Hydrolab.  Surface salinity data recorded in the field also corresponded well with the AutoSal data.  Bottom salinities recorded in the field with the Hydrolab and CTD, however, were an average of 4 ppt higher than those recorded in the lab with the Hydrolab and AutoSal using water taken from the horizontally-mounted bottom-tripping Niskin.  Lab values ranged between 1.0 and 6.7 ppt lower than field samples.

Since the proper functioning of the Hydrolab was supported by the AutoSal and the CTD, the water from the horizontal Niskin does not appear to have been collected at the bottom.  Smaller discrepancies (< 1 ppt) could be expected due to the Niskin not sampling at exactly the same location as the other instruments.  Differences of > 3 ppt, however, are highly unlikely.  Water column data suggest that the horizontally-mounted Niskin sample salinities would have been encountered above the bottom.

The horizontal Niskin was used to collect bottom water samples at all stations, but bottom salinity samples were collected only for transect C.  The possibility exists that other bottom samples also had salinities that differed from those recorded by the Hydrolab or CTD.  The "bottom" samples may not represent bottom water.