Methodology for Underway Bottle Data

L.A. Codispoti (codispot@hpl.umces.edu)

Horn Point Laboratory, January 2001

General Comments:

This "readme" file pertains to underway salinity, dissolved oxygen, and nutrient
data taken from the Research Icebreaker N.B.Palmer during cruise 97-08
(November-December 1997)using the ship's...  Dr. Walker Smith of the Virginia
Institute of Marine Sciences (wos@vims.edu) was the chief scientist during this
fourth process study leg of the U.S. JGOFS program in the Southern Ocean
(AESOPS).

THESE FILES REPRESENT A DATA RESCUE EFFORT BASED ON THE EXAMINATION OF HAND
WRITTEN SAMPLING LOGS THAT WERE IN MANY CASES INADEQUATELY FILLED OUT.  WE
SOMETIMES HAD TO MAKE INFORMED DECISIONS BASED ON OUR KNOWLEDGE OF THE SHIP'S
TRACK AS TO WHETHER LONGITUDES WERE EAST OR WEST.  SIMILARLY, IT WAS NOT ALWAYS
CLEAR WHETHER OR NOT TIMES AND DATES WERE IN UTC (ZULU), AND WE DID OUR BEST TO
STRAIGHTEN THEM OUT.  THE SEASOAR DATA FROM NOV. 25 AND 26 ARE ASSUMED TO COME
FROM THE SHIP'S UNDERWAY PUMPING SYSTEM AS WE ASSUMED FOR ALL OF THESE UNDERWAY
SAMPLES.  WE HAVE PERFORMED NO EDITING ON THE THERMOSALINOGRAPH (TSG)
TEMPERATURES AND SALINITY VALUES.  WE HAVE SIMPLY ENTERED THE VALUES THAT WERE
HAND WRITTEN ON THE SAMPLE LOGS.

No units are given for salinity in this report because the most recent
definitions of salinity define it as a dimensionless number.  Winkler oxygen
values are reported in ml/l. The Nutrient values are reported in micromolar.
They can be converted to micromoles per kg, by combining laboratory temperature
on the Palmer (approx. 21 deg C during this leg) and the salinity of the sample
to compute density and then dividing the value in micromolar by this number.


Methods:

In general, the methods employed for the bottle salinity, Winkler dissolved
oxygen, and nutrient analyses did not differ significantly from those described
in the JGOFS protocols that were distributed in 1994 (UNESCO, IOC Manual and
Guide #29). Minor differences included the following: 1) The weights of the
salts used for primary standards for dissolved oxygen were not adjusted to an
"in vacuo" basis as suggested in the protocols. It is unlikely that this
departure from procedure would cause significant errors. Our calculations
suggest that the maximum differences arising from our decision to not correct to
an "in vacuo" basis would be 0.02%.  2) The protocols give one a choice of
adjusting nutrient methods so that calibration curves are strictly linear, or
opting for more response and taking into account non-linearities.  We choose the
former method. 3) No corrections were made for "carryover" between nutrient
samples run on the Technicon Autoanalyzer. Carryover effects in our nutrient
analyses are generally less than ~2% of the concentration difference between
adjacent samples, and were minimized by arranging samples in depth order and by
running duplicate samples in some cases.  4) Calibration and re-calibration of
volumetric ware were not exactly as described in the JGOFS protocols.  5)
Duplicate oxygen samples were not routinely drawn. 6) The JGOFS protocols do not
describe an automated technique for the analysis of ammonium concentrations.  We
employed the Berthelot reaction using a method somewhat similar to the method
described by Whitledge et al. (1981, Whitledge, T.E., Malloy, S.C., Patton, C.J.
and Wirick, C.D. Automated Nutrient Analyses in Seawater. Brookhaven National
Laboratory Rept. BNL 51398, 216 pp.).  Details about the ammonium method and
about other aspects of the nutrient analyses protocols may be obtained by
contacting Dr. Louis I. Gordon of Oregon State University (lgordon@oce.orst.edu)


Thermosalinograph (TSG) Temperature and Salinity:

We have not edited the Thermosalinograph data written on the log sheets.  It is
our experience that bottle salinities are more reliable and accurate than TSG
values. Consulting the CTD data that have been previously submitted may give
some insight into the accuracy of the TSG data.  The interested user may also
wish to contact Dr. John Morrison to learn more about the TSG data
(John_Morrison@ncsu.edu).


Salinity:

Salinities were determined with Guildline Autosal salinometers. New vials of
standard sea-water were used to standardize before and at the end of every run.


Dissolved oxygen:

The Winkler dissolved oxygen apparatus was built and supplied by the SIO/ODF
group.  This system is computer controlled and detects the end-point
photometrically.  Temperatures of the thiosulfate and standard solutions are
automatically monitored by this system.


Nutrients:

Note that the terminology used to describe nutrients has become somewhat loose
over the years and that silicate = silicic acid, dissolved silicon or reactive
silicate, and phosphate = reactive phosphorus. Nutrient analyses were performed
on a 5-channel Technicon II AA system that was modified and provided by Dr. Lou
Gordon of Oregon State University.

During PALMER leg 96-04a, the nutrient standards provided by Dr. Gordon's group
were compared with standards from the Ocean Data Facility Group at Scripps and
with standards purchased from Ocean Scientific International (OSI). Interested
parties may contact Dr. Louis I. Gordon (lgordon@oce.orst.edu) if they are
interested in the details of these inter-comparisons.  The only notable
differences were a tendency for the OSI silicate signals to be ~1% high relative
to the OSU standards, and OSI nitrite standard signals to be 4% low relative to
OSU nitrite standards.  We believe that the OSI nitrite standards were in error,
but in any event, nitrite concentrations in the water column observations rarely
exceeded 0.25 micromolar, so any errors arising from systematic errors in
standards are likely to be less than 0.01 micromolar.  Because of the
concentration of observations over the Ross Sea Shelf, significant amounts of
nitrite and ammonium could occur throughout the water column, and the usual
method of looking at deep-water values to correct nitrite and ammonium baselines
could not be employed.  Thus, there may be some baseline uncertainties in
nitrite and ammonium concentrations that would induce errors of less than ~0.1
micromolar for ammonium and less than ~0.02 micromolar for nitrite.  Tests of
the salt effect in the ammonium analysis during this cruise suggested that it
was negligible for the water column values reported here. Cd column efficiencies
appeared to be 97% or greater throughout this leg.  Because of the high Cd
column efficiencies and low water column nitrite values (generally less than
0.25 micromolar), it was not necessary to correct for Cd column efficiency when
subtracting nitrite values from the nitrate plus nitrite analyses to obtain
nitrate concentrations.


Queries:


Questions about these data may be addressed to:
Dr. L.A. Codispoti
Horn Point Laboratory
University of Maryland Center for Environmental Studies
Cambridge, MD 21613-0775
codispot@hpl.umces.edu