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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. When cases
of a larger carryover effect could be determined, they are noted in
the cast specific comments. 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 cruise TN045. 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.
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 20 seconds before
tripping. This value was based on data obtained during TN039 when the
rosette was equipped with fewer electronic packages. During this
cruise (TN043), a decision was made to increase soak times to 30
seconds or until the deck read-outs stabilized because differences
between bottle salinities and the values obtained by the CTD when the
bottles were tripped were, in some cases, larger than anticipated. The
bottles were probably flushing relatively rapidly but it was noted
that the companion CTD data sometimes continued to change for periods
longer than 20 seconds. This was probably because of the additional
equipment mounted near the CTD sensors during TN043. This equipment
can act as a heat source/sink and interfere with flushing and
equilibration of the CTD sensors on the up cast. This adjustment was
made approximately mid-way through TN043. Whether 20 second soak
times were the cause of some of the differences has not been
determined. The cast specific comments notes those instances
where agreement between bottle and CTD salinities was greater than
expected.
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 and in the cases that have
been noted above and mentioned in the headers for individual
casts. More information on the quality of the salinity data are given
in the companion CTD report.
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. An
independent "Sagami" standard was compared with a SIO/ODF primary
standard. The agreement between these standards was +-0.02 per cent.
These standards were made up at sea with independent volumetric
ware. The linearity of the "Dosimat" automatic buret was also checked
during this cruise.
NOTE THAT THE TWO LAST DECIMAL PLACES ARE MEANINGLESS IN THE COLUMNS
THAT EXPRESS DISSOLVED OXYGEN IN mM and in mM/kg.
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 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. The three Eppendorf
maxipettors used to make the routine standard dilutions were
calibrated at Old Dominion University after cruise TN045. For the
three maxipettors and three tips that were returned for
re-calibration, the largest departure from the nominal values was 0.5%
at 2.50 ml. For the 5.00ml range, used to make the working standards,
the "worst" of these maxipettors (with its companion tip) was "off" by
0.2%, and the agreement between dialed and calibrated values for all
three instruments/tips was better than 0.1% at the 7.50 and 10.00 ml
settings. The 2.50, 7.50 and 10.00 ml settings were used for weekly
determinations of the linearity of each nutrient analysis.
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 towards the end of the cruise. The efficiencies were
all greater than 96.7% and frequently close to 100%. Corrections have
been made that should reduce any errors in nitrate arising from
deviations in cadmium column efficiency to less than 0.1 micromolar in
nitrite even for cases where nitrite concentrations concentrations
were maximal and Cd column efficiencies were minimal.
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.
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