AMT16 Nutrient (micro- and nano-molar) measurements from CTD bottle samples
Originator's Protocol for Data Acquisition and Analysis
Water samples were taken from the Sea-Bird CTD rosette system. The micro-molar samples were collected into acid-clean 60 ml HDPE (nalgene) sample bottles and the nano-molar samples into 120 ml HDPE (nalgene) sample bottles. Analysis for nutrients was completed within 2 hours for micro-molar and 3 hours for nano-molar measurements in all cases. Clean handling techniques were employed to avoid contamination of the samples. No underway samples were processed for this cruise.
The main nutrient analyser was a 5-channel Bran and Luebbe AAIII segmented flow autoanalyser. The analytical chemical methodologies used were according to Brewer and Riley (1965) for nitrate, Grasshoff (1976) for nitrite, Kirkwood (1989) for phosphate and silicate, and Mantoura and Woodward (1983) for ammonium.
Nanomolar ammonium concentrations were obtained using an adapted method from Jones (1991); this uses a fluorescent analysis technique following ammonia gas diffusion out of the samples, passing across a hydrophobic Teflon membrane due to differential pH chemistry. The ammonia analyser had several days of downtime and the recorder trace was intermittently too noisy to accurately determine nano-molar ammonia and the data were not supplied to BODC.
Nanomolar nitrate+nitrite, nitrate and phosphate concentrations were obtained on some samples using a 3-channel nanomolar analyser. This method combines sensitive segmented flow colorimetric analytical techniques with a Liquid Waveguide Capillary Cell (LWCC). The nitrate and nitrate systems worked throughout the cruise but due to time constraints the phosphate system was only used in the Southern Gyre region and beyond.
References Cited
Brewer P.G. and Riley J.P., 1965. The automatic determination of nitrate in sea water. Deep-Sea Research, 12, 765-772.
Grasshoff K., 1976. Methods of seawater analysis. Verlag Chemie, Weiheim: 317 pp.
Jones R.D., 1991. An improved fluorescence method for the determination of nanomolar concentrations of ammonium in natural waters. Limnology and Oceanography, 36, 814-819.
Kirkwood D.S., 1989. Simultaneous determination of selected nutrients in seawater. ICES CM1989/C:29, 12pp.
Mantoura R.F.C. and Woodward E.M.S., 1983. Optimisation of the indophenol blue method for the automated determination of ammonia in estuarine waters. Estuarine, Coastal and Shelf Science, 17, 219-224.
BODC Data Processing Procedures
The micro- and nano-molar nutrient data were submitted to BODC in a Microsoft Excel file. Sample metadata were checked against each dataset and the information held in the database. There were micro-molar ammonium but no nano-molar ammonium data provided to BODC. One set of micro- and nano-molar data was provided for a bottle that reportedly did not fire. These data were not loaded into the database.
There were a few discrepancies between information from the Sea-bird log files (used by BODC as a main reference for CTD rosette bottle metadata information) and the data originator records regarding the depth of the bottle firing for micro and nano-molar nutrient data (see table in the Problem Report section below). In particular for cast 43 a number of bottles had an offset of ~11 m between the actual depths sampled and the depths provided in the originator's data files.
Data from the LWCC systems were submitted in units of nmol l -1 . Nano-molar data were divided by 1000 to convert the units to µmol l -1 for storage in the database. Users should be aware that these LWCC measurements are valid to the fourth decimal place. The data were assigned parameter codes defined in BODC parameter dictionary. Data loaded into BODC's database using established BODC data banking procedures.
A parameter mapping table is provided below;
| Originator's Parameter | Units | Description | BODC Parameter Code | Units | Comments |
|---|---|---|---|---|---|
| Ammonium (AAIII) | µmol l -1 | Concentration of ammonium {NH 4 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | AMONAATX | µmol l -1 | - |
| Nitrate+Nitrite (AAIII) | µmol l -1 | Concentration of nitrate+nitrite {NO 3 +NO 2 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | NTRZAATX | µmol l -1 | - |
| Nitrate+Nitrite (LWCC nano-molar system) | nmol l -1 | Concentration (nM sensitivity) of nitrate+nitrite {NO 3 +NO 2 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis with liquid waveguide capilliary cell | NTRZLWTX | µmol l -1 | nmol l -1 converted to µmol l -1 (conversion used * 1/1000) |
| Nitrite (AAIII) | µmol l -1 | Concentration of nitrite {NO 2 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | NTRIAATX | µmol l -1 | - |
| Nitrite (LWCC nano-molar system) | nmol l -1 | Concentration (nM sensitivity) of nitrite {NO 2 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis with liquid waveguide capilliary cell | NTRILWTX | µmol l -1 | nmol l -1 converted to µmol l -1 (conversion used * 1/1000) |
| Phosphate (AAIII) | µmol l -1 | Concentration of phosphate {PO 4 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | PHOSAATX | µmol l -1 | - |
| Phosphate (LWCC nano-molar system) | nmol l -1 | Concentration (nM sensitivity) of phosphate {PO 4 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis with liquid waveguide capilliary cell | PHOSLWTX | µmol l -1 | nmol l -1 converted to µmol l -1 (conversion used * 1/1000) |
| Silicate (AAIII) | µmol l -1 | Concentration of silicate {SiO 4 } per unit volume of the water body [dissolved plus reactive particulate phase] by colorimetric autoanalysis | SLCAAATX | µmol l -1 | - |
Data Quality Report
The dataset has been checked by the data originator - any suspect data values were removed from the data set before submission to BODC.
Measurement precision information from data originators
The detection limits for measurements from the AAIII Bran and Luebbe autoanalyser have are 0.02 µmol l -1 , except the colorimetric ammonium which has a detection limit of 0.03 µmol l -1 . Samples in the database with a flag of "<" had concentrations below the specified detection limits.
At low concentrations, the values obtained by the LWCC are likely to be more accurate than those from the AAIII analyser.
Problem Report
Because of a few remaining uncertainties in matching sample measurements with SeaBird bottle firing depths, users should exert caution when using data from the following CTD cast and depth (differecnes marked by asterisk are believed to have been caused by a typographical error):
| CTD Cast ID | Originator Bottle No. | Originator Sample Depth | BODC Bottle No. | BODC Depth | BODC Bottle ID |
|---|---|---|---|---|---|
| 35 | 3 | 300 | 3 | 350 | 515692 |
| 43 | 20 | 31 | 20 | 42.6 | 516501 |
| 43 | 18 | 57 | 18 | 68.6 | 516500 |
| 43 | 6* | 75 | 16 | 88.2 | 516498 |
| 43 | 15 | 100 | 15 | 112.1 | 516497 |
| 43 | 14 | 120 | 14 | 131.7 | 516496 |
| 43 | 12 | 130 | 12 | 141.9 | 516494 |
| 43 | 5 | 140 | 5 | 151.5 | 516493 |
| 43 | 10 | 180 | 10 | 191.1 | 516492 |
| 43 | 9 | 225 | 9 | 236.7 | 516491 |
| 43 | 8 | 300 | 8 | 311.7 | 516490 |
