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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. 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. WE HAVE NOT YET RECALIBRATED THE VOLUMETRIC WARE USED DURING
TN054. WE WILL UPDATE THE DATA IF RECALIBRATION SUGGESTS A NEED TO DO
THIS, BUT WE DO NOT EXPECT SIGNIFICANT CHANGES 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. ON LEGS PRIOR TO THIS ONE, OXYGEN STANDARDIZATIONS WERE
RUN USING REAGENTS THAT DID NOT CONTAIN AZIDE, BUT DISCUSSIONS AND
TESTS SUGGESTED THAT IT WOULD BE BETTER TO STANDARDIZE WITH AZIDE,
DESPITE SOME CONFUSION IN THE LITERATURE ON THIS MATTER.
CONSEQUENTLY, WE SWITCHED PROCEDURES BEGINNING WITH LEG TN053
AND USED REAGENTS CONTAINING AZIDE TO STANDARDIZE. OUR TESTS SUGGEST
THAT THE MAXIMUM CHANGE IN OXYGEN CONCENTRATIONS ARISING FROM THIS
CHANGE WOULD OCCUR AT THE HIGHEST OXYGEN CONCENTRATIONS AND BE < ~0.01
ML/L.
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 1.01 DECIBAR DIFFERENCE BETWEEN CTD SENSOR AND SAMPLING BOTTLE
POSITIONS.
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. Agreement between bottle salinities and the recently
calibrated sensors on the Sea Bird CTD systems was usually better than
0.02 (except in regions of strong gradients) before post-cruise data
processing which employs the bottle salinities to correct the CTD
salinities. 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. A
similar check made during TN054 suggested agreement of better than
+-0.15 per cent. The linearity of the "Dosimat" automatic buret was
also checked during cruises TN043 and TN054 with good results.
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 poll of PI's and were
intended to cover the full depth concentration range for the Arabian
Sea. Starting with TN050 the nitrite range was expanded to
0-7micromolar because we found maximum nitrite concentrations to be
~6.5 micromolar.
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 "
On TN043, the volumetric equipment used for making routine nitrate and
phosphate standards was checked against volumetric ware calibrated by
LAC. The average difference between these comparisons of mid-range
standards was + or - 0.2% for phosphate and + or -0.4% for nitrate.
Because nitrite values in the suboxic waters of the Arabian Sea can
attain values of approximately 5 micromolar and because our routine
standards contained 22.5 micromoles of nitrate and 2.5 micromoles of
nitrite, we kept track of the efficiency of the Cd column that
reduces nitrate to nitrite in the nitrate analysis. The lowest column
efficiency determined on this cruise was 97.5%. 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 7 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 (5.0
micromolar) 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 the first JGOFS Arabian Sea process study cruise
(TN043) suggested that the ammonium signal decreases by approximately
3.5% for a salinity increase of 1.00. Comparisons of an independent
standard prepared by LAC with an 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. Comparisons of independent high concentration ammonium
standards (~2.5 and 5.0 micromolar) prepared by LAC with SIO standards
during TN054 agreed to better than + - 1% for four out of the five
standards when corrected for a salinity effect of 4.5%/1.00S on that
cruise. One standard agreed to only + - 2.5%, but we assume that this
was due to a dilution error. We believe that the suite of ammonium
comparisons suggests no systematic differences arising from standards
and dilutions, as all of the differences are within the precison of
the ammonium analysis. Our 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
(TN054) THE SALINITY EFFECT CORRECTION IS A 4.5% DECREASE IN SIGNAL
FOR A SALINITY INCREASE OF 1.00. The average salinity of the working
standards used to calibrate the ammonium method was ~34.96 for casts
TN05400101-TN05401302 (inclusive), ~35.27 for casts
TN05401303-TN05401902 (inclusive), 35.14 for casts
TN05401903-TN05402601 (inclusive), and 34.59 for the remainder of the
casts. The ammonium method has additional problems, such as
contamination of "baseline" water etc. These problems can introduce
inaccuracies on the order 0.1 micromolar, so differences in ammonium
concentrations of less than ~0.1 micromolar should not be
over-interpreted.
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