D242 CTD Discrete Nutrient Sampling Document
Originator's Protocol for Data Acquisition and Analysis
Sample collection
Discrete dissolved inorganic nutrient samples were provided from 109 CTD casts performed during the RSS Discovery cruise 242. Dissolved silicon (referred to as silicate and reported as SiO 3 ), nitrate and nitrite (referred to as nitrate or NO 2 +NO 3 ) and phosphate (PO 4 ), were collected after oxygen samples had been drawn from the Niskin bottles. Samples were collected into 30 ml "diluvial" sample cups, and rinsed 3 times before filling. The samples were then stored in a refrigerator at 4°C until analysed, 1 to 12 hours after collection.
In addition, a deep water sample was collected from test station 13630 at ~ 1915 m for calibration purposes.
Sample analysis
The samples were analysed onboard using a ChemLab AAII type Auto-Analyser coupled to a Digital-Analysis Microstream data capture and reduction system. The primary calibration standards for dissolved silicon, nitrate and phosphate were prepared from sodium hexaflurosilicate, potassium nitrate and potassium dihydrogenn phosphate respectively.
A number of samples were analysed in duplicate. This was a limited due to the large volume of samples required to be analysed daily and the instability of the photometer which experienced sensitivity to vibrations in particular during use of the CTD winch system. Where duplicate samples were taken, a definitive data value was derived from the averaging of the two samples. There were instances were one of the duplicated samples were deemed of poor quality by the Originator and so in these cases only the good quality data values has been used.
Silicon
Dissolved silicon analysis followed the standard AAII molybdate-ascorbic acid method with the addition of a 37°C heating bath (Hydes, 1984).The colorimeter was fitted with a 50 mm flow cell and a 660 nm filter. The gain was adjusted to 2.9 for a maximum response at 40.
Nitrate
Nitrate (and nitrite) analysis followed the standard AAII method using the sulphanimamide and naphtylethylenediamine-dihydrochloride with a copperised-cadmium filled glass reduction column. A 15 mm flow cell and 540 nm filter was used. The gain was adjusted to 2.9 for a maximum response at 40 µmol l -1
Phosphate
Phosphate analysis followed the standard AAII method which adheres to the method of Murphy and Ripley (1954). A 50 mm flow cell and 880 nm filter were used. The gain was set to 9 for concentrations of up to 2 µmol l -1
Further details are provided in the cruise report for cruise D242 .
BODC Data Processing Procedures
The nutrient data were supplied to BODC in Pstar format containing duplicate data and corrected nutrient data for silicon, nitrate and phosphate.The Originator supplied flagged channels for each of the corrected variables populated by quality flags as defined by WOCE Hyrographic Programme. These files were reformatted to ASCII format by BODC, and the corrected nutrient values and any associated flags were extracted and loaded into BODC's database under the ORACLE Relational Database Management System. Any further data that BODC considered unrealistic were flagged suspect.
Content of data series
| Originator's Parameter | Unit | Description | BODC Parameter code | BODC Unit | Comments |
|---|---|---|---|---|---|
| SiO 3 | µmol l -1 | Concentration of silicate {SiO4} per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | SLCAAATX | µmol l -1 | No unit conversion necessary |
| NO 2 +NO 3 | µmol l -1 | Concentration of nitrate+nitrite {NO3+NO2} per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | NTRZAATX | µmol l -1 | No unit conversion necessary |
| PO 4 | µmol l -1 | Concentration of phosphate {PO4} per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | PHOSAATX | µmol l -1 | No unit conversion necessary |
References
Cunningham, A., (2000). RSS Discovery Cruise 242, 07 SEP - 06 OCT 1999. Atlantic - Norwegian Exchanges. Cruise Report No. 28 Southampton Oceanography Centre.
Hydes, D. J., (1984). A manual of methods for the continuous flow determination of ammonia, nitrate-nitrite, phosphate and silicate in seawater . Wormley, UK, Institute of Oceanographic Sciences, 37pp. (Institute of Oceanographic Sciences Report 177).
Murphy, J. and Ripley, J. P., (1954). A modified single solution method for the determination of phosphate in natural waters. Anal. Chem. Acta., 27, 31-66.
