ࡱ > M O L ` T! bjbj *@ T H H H H H H H \ \ \ ! ( ^ &! (! (! (! (! (! (! $ ]" h $ Z L! H L! H H a! H H &! H H ( X 4 w! 0 ! % % % H < > , $ L! L! ! \ \ \ \ \ \ \ \ \ H H H H H H Slveig R. lafsdttir# and Jn lafsson*#
University of Iceland* and Marine Research Institute#, Reykjavk.
Determinations of nutrient concentrations onboard R/V G. O. Sars, Cruise 200311
Introduction.
The determination of sea water nutrient concentrations on this cruise served two purposes:
Characterization and identification of water masses.
Description of biochemical processes and activity.
In both cases the nutrient data complement other measurements.
Instrument and methods
Samples were collected in 250 ml soft low density polyethylene acid washed bottles. They were kept refrigerated until analysis if it did not commence immediatly after collection.
Subsamples from the surface layer, 0-60 m, were syringe filtered through a 0.45m Whatman PURADISC filter to avoid turbidity blank effects, particularly on phosphate. Subsamples from deeper water were not filtered.
A Chemlab three channel autoanalyzer was used, set up for determinations of dissolved phosphate, nitrate and silicate. The methods were those described by Grasshoff ADDIN EN.CITE Grasshoff197078078K. Grasshoff1970A simultaneous multiple channel system for nutrient analysis in seawater with analog and digital data recordTecnicon QtrlyTechnicon Quarterly37-17water, Si, NO3, NO"Analytical(Grasshoff, 1970) except for phosphate were a modified version of the Murphy and Riley method was automated ADDIN EN.CITE Murphy196286086J. MurphyJ.P. Riley1962A modified single solution method for the determination of phosphate in natural watersAnal. chim. ActaAnalytica chimica Acta2731-36water, PAnalytical(Murphy and Riley, 1962). A series of 5 working standards was prepared with each batch of samples. The data was recorded on a three channel recorder and processed manually. The measured volumetric concentrations were converted to a mass basis using the CTD sample salinities and the EOS 80. For this calculation the sample temperature was set 15C, which approximated the laboratory conditions.
Quality control
To assess the accuracy of our methods and procedures we have for many years participated and subscribed to the QUASIMEME laboratory QC programme ADDIN EN.CITE Wells19974910491 Wells, David E.Aminot, Alain De Boer, Jacob Cofino, Wim Kirkwood, Don Pedersen, Britta1997A review of the achievements of the EU project ‘QUASIMEME’ 1993–1996Marine Pollution Bulletin351-63-17(Wells et al., 1997). Immediately after the G. O. Sars cruise, 3 samples (QNU116SW-QNU118SW) from QUASIMEME with unknown concentrations were measured and reported on as in the following table:
QNU116SWQNU117SWQNU118SWNitrate+nitrite ( in mole l-1)
assigned value
measured at MRI
difference
10.53
10.67
-0.14
5.70
5.97
-0.27
0.12
0.05
0.07Phosphate ( in mole l-1)
assigned value
measured at MRI
difference
1.20
1.20
0.00
0.75
0.75
0.00
0.37
0.39
0.02Silicate ( in mole l-1)
assigned value
measured at MRI
difference
4.14
4.51
-0.37
2.72
2.74
-0.02
0.39
0.39
0.00
In addition, a laboratory reference material, LRM ADDIN EN.CITE Aminot19984900490Aminot, A.Kérouel, R.1998Pasteurization as an alternative method for preservation of nitrate and nitrite in sea water samplesMar. Chem.61203-208Analytical(Aminot and Krouel, 1998), is regularly produced and compared to QUASIMEME materials. On this cruise we had LRM Batch V, with the following nominal concentrations: Phosphate 0.81 mol/l, nitrate 10.9 mol/l and silicate 9.9 mol/l. The LRM was run with each station. Results from these measurements are indicative of the precision and accuracy of the onboard procedures. The mean concentrations measured and standard deviations were as follows: Phosphate 0.80 mol/l (s.d. 0.02; n=134), nitrate 10.91 mol/l (s.d. 0.13; n=133) and silicate 10.27 mol/l (s.d. 0.27; n=144). A correction based on the LRM results was only applied in one instance, for Si at one station.
The influence of the rosette sampling and sample drawing was assessed by closing 22 water bottles at 1000 m depth at station 188. The mean nutrient results and the standard deviations were as follows:
St. 188Nitrate+nitrite ( in mole l-1)
sd
n13.54
0.03
22Phosphate ( in mole l-1)
sd
n0.86
0.02
22Silicate ( in mole l-1)
sd
n7.91
0.07
22
References
ADDIN EN.REFLIST Aminot, A. and Krouel, R., 1998. Pasteurization as an alternative method for preservation of nitrate and nitrite in sea water samples. Mar. Chem., 61: 203-208.
Grasshoff, K., 1970. A simultaneous multiple channel system for nutrient analysis in seawater with analog and digital data record. Tecnicon Qtrly, 3: 7-17.
Murphy, J. and Riley, J.P., 1962. A modified single solution method for the determination of phosphate in natural waters. Anal. chim. Acta, 27: 31-36.
Wells, D.E. et al., 1997. A review of the achievements of the EU project QUASIMEME 19931996. Marine Pollution Bulletin, 35(1-6): 3-17.
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