Cast specific comments, quality assessment, analytical methods as prepared
by L. Codispoti
FOR THE HYDROGRAPHIC BOTTLE DATA
L.A. Codispoti (lou@ccpo.odu.edu)
Old Dominion University, November 1995
General Comments:
This "readme" file pertains to the salinity, dissolved oxygen, and
nutrient data taken from sampling bottles with the hydrographic
rosette that was typically equipped with 24 10-liter Niskin type
bottles during RV T.G. Thompson cruise TN045. This cruise was the
second JGOFS Arabian Sea Process Leg and took place during March-April
1995. Dr. John Marra of the Lamont Doherty Earth Observatory
(marra@ldeo.columbia.edu) was the chief scientist. DATA TAKEN WITH THE
CLEAN ROSETTE USED FOR OBTAINING PRIMARY PRODUCTION AND OTHER TYPES OF
BIOLOGICAL SAMPLES ARE NOT INCLUDED IN THIS REPORT.
Some questionable data are not included in this report. These data are
still retained in files at Old Dominion University, and are available
upon request.
No units are given for salinity in this report because the most recent
definitions of salinity define it as a dimensionless number. To
accommodate every preference, Winkler oxygen values are reported in
ml/l, micromolar and micromoles per kg. The latter values can only be
calculated with a knowledge of the oxygen sample temperatures when the
samples were drawn. These "draw temperatures" are not reported here,
but can be obtained by contacting lou@ccpo.odu.edu. Nutrient values
are reported in micromolar. They can be converted to micromoles per
kg, by combining lab. temperature on the Thompson (approx. 23.5 deg C)
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
June, 1994. Minor differences included the following: 1) Sea Bird CTD
systems and bottle carousels were employed (SBE- 9+ underwater units,
SBE-11 deck units, SBE-32 carousels). These units represent a newer
generation of equipment than the units described in the JGOFS
protocols. 2) The weights of the salts used for primary standards for
dissolved oxygen and nutrients 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 range from 0.02%
(oxygen standards) to 0.06%(ammonium standards). 3) 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 latter method. 4) No
corrections were made for "carryover" between nutrient samples run on
the Technicon Autoanalyzer. Data from this cruise, suggest that
carryover effects in our nutrient analyses are generally less than ~2%
of the concentration difference between adjacent samples. Examination
of cases where more than one sample was taken from a depth at which
there was a significant increase in nutrient concentrations will help
the user determine the carryover effect for many individual casts. 5)
Calibration and re-calibration of volumetric ware were not exactly as
described in the JGOFS protocols, but this was largely compensated for
by comparing independent standards diluted with independent volumetric
ware, and by re-calibration of some of the volumetric ware after
cruises TN045 and TN050. DATA FOR THIS CRUISE (TN045) HAVE BEEN
CORRECTED FOR ERRORS IN THE PIPETS BY MULTIPLYING THE NITRATE AND
PHOSPHATE VALUES BY 0.998, THE SILICATE VALUES BY 0.999, THE NITRITE
VALUES BY 1.004 AND THE AMMONIUM VALUES BY 1.003. 6) Duplicate oxygen
samples were not drawn from every Niskin or Go-Flo bottle, but there
were several comparisons of bottles tripped at the same depth. 7)
Azide was added to the Winkler oxygen pickling reagents to destroy
nitrite that can be present in relatively high concentrations in the
Arabian Sea.
Pressure:
There was a change in pressure sensors during this cruise:
Station 04500101 --- Station 04501003 used SeaBird Pressure Sensor 34901
Station 04501101 --- Station 04503002 used SeaBird Pressure Sensor 43434
It turns out that Sensor 43434 had a pressure hysteresis problem. This problem
appears to be linear. In order to correct the upcast pressures, the following
method was used:
1. A nominal "surface" pressure was computed for the CTD using the
mean surface pressure for the previous cruise which was
2.2 db +/- 1.0 db. (If we do this for the first 10 stations
on TN045, the comparible value was 2.3 +/- 1.0 db).
We assume that this is the nominal surface pressure reading
from the pressure sensor when the CTD package is just
below the surface of the water.
2. Also, the pressure offset for the deepest station (04501311)
was 7.4 meters and the station depth was 4300 meters. When we
look at the pressure offsets for all of the stations, they
linearily increase as a function of maximun station depth, with
a slope of .0017 (for example for station 04501311 the offset
was -7.2 meters).
3. This correction was applied in the following manner:
Corrected Pres =
(Max Pres for Station - Original Bottle Pres) * .017
+ Original Bottle Pres
4. Using this method, the accuracy of the corrected pressure is on the
order of +/- 1 db.
John M. Morrison
Dept of Marine, Earth and Atmospheric Sciences
North Carolina State University
1125 Jordan Hall --- Box 8208
Raleigh, NC 27695 - 8208
Phone: (919) 515-7449
Fax: (919) 515-7802
Email: John_Morrison@NCSU.EDU
Temperature:
The temperature data associated with each bottle depth were taken by
the CTD system during the bottle tripping process. Consult the
companion CTD data report for this cruise to learn more about the CTD
system.
Sampling:
The samples in this report were taken from 10 liter Niskin
bottles.
Because there is little or no lag time between triggering a bottle and
bottle closure with the new SeaBird rosette systems, bottles were
generally held at the sampling depth for at least 30 seconds before
tripping or until the deck read-outs stabilized if this took more than
30 seconds.
NOTE THAT THE MID-POINT OF THE SAMPLING BOTTLES WAS ONE METER ABOVE
THE CTD SENSORS. THE DATA HAVE NOT BEEN CORRECTED FOR THIS ONE METER
OR 1.1 DECIBAR DIFFERENCE BETWEEN CTD SENSOR AND SAMPLING BOTTLE
POSITIONS.
Salinity:
Salinities were run on almost every bottle sample with new vials of
standard sea-water used before and at the end of every run (12-36
samples). Agreement between bottle salinities and the recently
calibrated sensors on the Sea Bird CTD systems was usually better than
0.01 except in regions of strong gradients. More information on the
quality of the salinity data are given in the companion CTD report.
Both the CTD salinity data at the time of bottle tripping and the
salinities run on the Niskin bottle samples with an Autosal salinometer
are reported here.
Dissolved oxygen:
The Winkler dissolved oxygen set-up was built and supplied by the
SIO/ODF group. This system is computer controlled and detects the
end-point photometrically. Temperature of the thiosulfate and
standard solutions is automatically monitored by this system. Checks
on cruises TN039 and TN043 between independent standards prepared with
independent volumetric ware gave agreement of +-0.02 per cent. The
linearity of the "Dosimat" automatic buret was also checked during
cruise TN043.
Nutrients:
Note that the terminology used to describe nutrients has become
somewhat loose over the years and that silicate=silicic acid, and
phospate=reactive phosphorus.
Nutrient analyses were performed on a 5-channel Technicon II AA
system that was modified and provided by the SIO/ODF group.
In assessing the nutrient standard comparisons outlined below,
note that the full-scale ranges for nutrients were as follows:
Ammonium =0 to 5 micromolar
Nitrate =0 to 45 "
Nitrite =0 to 5 "
Phosphate =0 to 3.6"
Silicate =0 to 180 "
These ranges were arrived at after an Internet pole of PI's and
cover the full depth concentration range for the Arabian Sea.
On the set-up and calibration cruise (TN039), the SIO/ODF nitrate and
nitrite standards and standards from the National Institute of
Oceanography in India (provided by S.W.A. Naqvi) were compared with
the following results:
NIO Nitrate Std.= 22.6 micromolar; SIO/ODF=22.5 micromolar
NIO Nitrite Std.= 2.42 micromolar; SIO/ODF = 2.50 micromolar
As can be inferred from the above, the nitrate plus nitrite
values were almost identical in the mixed standards;
25.02 (NIO) vs 25.00 (SIO)micromolar.
On TN039 Lou Codispoti prepared independent primary nitrate, nitrite,
silicate and phosphate standards for comparison with SIO/ODF primary
standards, and made dilutions using glassware entirely independent of
the SIO/ODF glassware.
The results were as follows:
Codispoti SIO/ODF
Nitrate 26.96 micromolar 26.85 micromolar
Nitrite 2.90 " 2.86 "
Silicate 86.4 " 85.8 "
Phosphate 2.36 " 2.36 "
All of the above results are within plus or minus 0.5% of the full
scale values, and with the exception of nitrite, the rest are within
plus or minus 0.2% of the full scale values. On TN043, the volumetric
equipment used for making routine nitrate and phosphate standards was
checked against volumetric ware calibrated by LAC. The average of the
results agreed to within +-0.1% of the full scale value for phosphate
and +-0.2% of the full scale value for nitrate.
Because nitrite values in the suboxic waters of the Arabian Sea can
attain values of approximately 5 micromolar, we kept track of the
efficiency of the Cd column that reduces nitrate to nitrite in the
nitrate analysis. The efficiencies were all greater than 98.8% and
frequently close to 100%. Therefore, no corrections have been made
for any errors in nitrate arising from deviations in cadmium column
efficiency. NOTE THAT THE FULL-SCALE NITRITE RANGE FOR THIS CRUISE WAS
5 MICROMOLAR AND THAT SOME CONCENTRATIONS EXCEEDED THIS VALUE. IN
THESE CASES, THE SAMPLES EITHER HAD TO BE DILUTED OR THE VOLTAGE RANGE
CHANGED ON THE RECORDER. THESE MANIPULATIONS TEND TO DEGRADE THE
ACCURACY OF NITRITE VALUES IN EXCESS OF 4.5 TO 5 MICROMOLAR.
The ammonium results are the least precise of all the nutrient
results. On TN039, three primary standards were compared with
agreement of about plus or minus three per cent of the full-scale
value. These standards may have agreed within the precision of the
method, but we found a significant salinity effect on the ammonium
results that might explain some of these differences since the
salinities of the comparison standards varied a bit. Experiments on
this first JGOFS Arabian Sea process study cruise (TN043) suggest that
the ammonium signal decreases by approximately 3.5% for a salinity
increase of 1.00. Comparisons of an independent standard compared by
LAC with the SIO standard on this cruise (TN043) when corrected for
salinity differences between the standards agreed to ~ + -0.1% of the
full-scale value. The largest absolute difference was 0.025
micromolar and the average difference was 0.013 micromolar for six
comparisons between 1-3 micromolar. Thus, the average difference
between these two independent standards was + -0.006 micromolar. These
results tend to confirm the need to take salinity differences between
samples and standards into account when calculating the final ammonium
concentrations. THE AMMONIUM VALUES IN THIS REPORT HAVE BEEN CORRECTED
FOR THIS EFFECT. ON THIS CRUISE THE SALINITY EFFECT CORRECTION IS A
3.2% DECREASE IN SIGNAL FOR A SALINITY INCREASE OF 1.00. On this
cruise, the salinity of the working standards used to calibrate the
ammonium method was 34.39.