AMT14 Nutrient (micro- and nano-molar) measurements from CTD bottle and underway samples
Originator's Protocol for Data Acquisition and Analysis
Water samples were taken from the Sea-Bird CTD rosette system and from the non-toxic supply tap. They were sub-sampled into acid-clean 60 ml HDPE (nalgene) sample bottles. Analysis for nutrients was completed within 3-4 hours of sampling in all cases. Clean handling techniques were employed to avoid contamination of the samples.
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 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 phosphate waveguide did not produce consistently reliable results.
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.
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 two separate Microsoft Excel files. Sample metadata were checked against each dataset and the information held in the database.
There were 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. The micro-molar nutrient data had the rosette bottle number provided, while the nano-molar nutrient data had the firing sequence number provided as the rosette bottle number. The micro- and nano-molar nutrient data were matched by the data originator based on sample ID and a final version supplied (Jan 2006). A few remaining depth discrepancies still remained and the data were loaded into the database by matching the originators' Niskin rosette position number to the depth recorded by the Sea-bird instrument. These data are listed in the table in the Problem Report section showing the originator's depth and the BODC database depth for the respective bottles and to which data sets this applied.
Data from the LWCC systems were submitted in units of nmol/l. Nano-molar data were divided by 1000 to convert the units to µmol/l 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 0.08 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:
CTD01: Data collected for CTD02 have depths matching the CTD01. However according to K Chamberlain CTD01 was cancelled. The UKORS log shows bottles were fired at station 01 the 30th of April and the cruise report mention that a previous predawn CTD was cancelled the 29th of April. Data loaded against CTD01
CTD named CTD40 by the originator matched the depths of CTD39 so it was loaded against CTD39 and CTD named CTD39 loaded against CTD038. The data originator agreed with the changes.
For the CTD 80 the originator depths did not match the Sea-Bird depth and therefore the data were loaded at BODC by matching the bottle rosette position numbers and the sample depths in the database are therefore currently different from that originally provided by the originator.
The table below summarises the records for which there was a significant discrepancy between the depth provided by the originator and the depth held in BODC's database:
| CTD Cast ID | Originator Bottle No. | Originator Sample Depth | BODC Bottle No. | BODC Depth | BODC Bottle ID | Micro or nano-molar dataset |
|---|---|---|---|---|---|---|
| AMT14_25 | 1 | 1000.00 | 1 | 716.3 | 511949 | Both |
| AMT14_74 | 17 | 19.00 | 18 | 23.5 | 510739 | Micro-molar |
| AMT14_74 | 14 | 34.00 | 14 | 43.6 | 510904 | Micro-molar |
| AMT14_74 | 12 | 65.00 | 12 | 79.5 | 510902 | Micro-molar |
| AMT14_74 | 9 | 80.00 | 9 | 99.5 | 510899 | Micro-molar |
| AMT14_74 | 7 | 90.00 | 7 | 109.8 | 510897 | Micro-molar |
| AMT14_74 | 4 | 120.00 | 4 | 150.1 | 510894 | Micro-molar |
| AMT14_80 | 20 | 10.00 | 20 | 6.9 | 510717 | Micro-molar |
| AMT14_80 | 17 | 19.00 | 17 | 12.8 | 510714 | Micro-molar |
| AMT14_80 | 14 | 34.00 | 14 | 24 | 510711 | Micro-molar |
| AMT14_80 | 12 | 65.00 | 12 | 34.6 | 510709 | Micro-molar |
| AMT14_80 | 9 | 80.00 | 9 | 49.8 | 510706 | Micro-molar |
| AMT14_80 | 7 | 90.00 | 7 | 65.2 | 510704 | Micro-molar |
| AMT14_80 | 4 | 120.00 | 4 | 80.6 | 510701 | Micro-molar |
