AMT13 Nutrient (micro- and nano-molar) measurements from CTD bottle and surface underway samples
Originator's Protocol for Data Acquisition and Analysis
Water samples were taken from the Sea-Bird CTD rosette system on most casts and from the non-toxic supply tap at 15:00 on days where concentrations were lower than the detection limit of the micro-molar analyses. The water samples were sub-sampled into acid-clean 60 ml HDPE (nalgene) sample bottles. Analysis for nutrients was completed within 3 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 samples were analysed for nitrate (Brewer and Riley, 1965), for nitrite (Grasshoff, 1976), phosphate and silicate (Kirkwood, 1989), and ammonium (Mantoura and Woodward, 1983).
Nanomolar ammonium concentrations were obtained with a fluorescent analysis technique following ammonia gas diffusion out of the samples, passing across a hydrophobic Teflon membrane due to differential pH chemistry (adapted from Jones, 1991).
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.
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
Data were submitted to BODC in Microsoft Excel spreadsheet format and saved to the BODC archive with accession number PML040070. Sample metadata were checked against information held in the database.
There were 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's records regarding the depth of the bottle firing. A number of these discrepancies could be resolved by identifying CTD rosette bottles that had misfired. This information was provided by the data originator and the bottles were flagged in the database. It was decided that unless obviously incorrect the data should be loaded into the database by matching the originators' Niskin rosette position number to the depth recorded by the Sea-bird instrument (see section "Problem Report" for details of affected stations and depths).
Data from the nanomolar ammonium and 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 | - |
| Ammonium (nano-molar system) | nmol l -1 | Concentration (nM sensitivity) of ammonium {NH 4 } per unit volume of the water body [dissolved plus reactive particulate phase] by nanomolar ammonium system after Jones (1991) | AMONNATX | µmol l -1 | nmol l -1 converted to µmol l -1 (conversion used * 1/1000) |
| 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:
CTD 24, 66.1 and 101 m: bottle 21 and 22 firing depth inverted in data originator as compared to Sea-bird (and database). The data were loaded by BODC by matching bottle sample depths and the rosette position numbers in the database are therefore currently different from that provided by the originator.
CTD 36, nano-molar nutrients supplied with depth typos for rosette position numbers 22 and 24 (provided 0.5 m for both where should be 500 m and 1000 m respectively as for the micro-nutrient data). The data were loaded by BODC by matching bottle rosette position numbers and the sample depths in the database are therefore currently different from that provided by the originator.
CTD 42, bottles from 50 m to 200 m (7,8, 9,10,11,12,13,14,18,19,21,22): mismatch in firing depth between Sea-bird and originator apparently due to bottles that did not fire. The data were loaded at BODC by matching bottle rosette position numbers and the sample depths in the database are therefore currently different from that originally provided by the originator.
CTD 53, 180.5 and 225.4 m: bottles 21 and 22 depth firing depth inverted in data originator as compared to Sea-bird (and database). The data were loaded at BODC by matching bottle rosette position numbers and the sample depths in the database are therefore currently different from that provided by the originator.
CTD 61, 1000 m: bottles 22 and 24 were both fired at ~1000 m according to SeaBird bottle firing information but attributed to 500 m and 1000 m by data originator. Nutrient concentrations clearly indicated that the samples originated from different depths so these data were loaded into the database according to the depths given by the originator.
CTD 67, 1000 m: bottles 22 and 24 were both fired at ~1000 m according to SeaBird bottle firing information but attributed to 500 m and 1000 m by data originator. Nutrient concentrations clearly indicated that the samples originated from different depths so these data were loaded into the database according to the depths given by the originator.
The table below provides a comparison between the depths in the source files and in BODC's database:
| CTD_cast | Sea-bird Bottle # | Sea-bird depth | Originator Bottle # | Originator Depth/notes | BODC ROSPOS | BODC Depth |
|---|---|---|---|---|---|---|
| CTD_24 | 21 | 66.1 | 21 | 101 | 21 | 66.1 |
| CTD_24 | 22 | 101 | 22 | 66.1 | 22 | 101 |
| CTD_42 | 8 | 50.5 | 8 | Did Not Fire | 8 | Misfired |
| CTD_42 | 9 | 60.8 | 9 | 50.5 | 9 | 60.8 |
| CTD_42 | 10 | 68.4 | 10 | 60.8 | 10 | 68.4 |
| CTD_42 | 11 | 70.4 | 11 | Did Not Fire | 11 | Misfired |
| CTD_42 | 12 | 74.1 | 12 | Did Not Fire | 12 | Misfired |
| CTD_42 | 13 | 76.3 | 13 | 74.1 | 13 | 76.3 |
| CTD_42 | 14 | 78.4 | 14 | 76.3 | 14 | 78.4 |
| CTD_42 | 18 | 80.3 | 18 | 78.4 | 18 | 80.3 |
| CTD_42 | 19 | 99.9 | 19 | 80.3 | 19 | 99.9 |
| CTD_42 | 21 | 150.4 | 21 | 99.9 | 21 | 150.4 |
| CTD_42 | 22 | 200.1 | 22 | 150.4 | 22 | 200.1 |
| CTD_53 | 22 | 180.5 | 22 | 225.4 | 22 | 180.5 |
| CTD_53 | 21 | 225.4 | 21 | 180.5 | 21 | 225.4 |
| CTD_61 | 22 | 1000.6 | 22 | 501.4 | 21 | 501.4 |
| CTD_61 | 24 | 1000.8 | 24 | 1000.8 | 24 | 1000.8 |
| CTD_67 | 22 | 1001.7 | 22 | 500 | 20 | 499.7 |
| CTD_67 | 24 | 1000.5 | 24 | 1000.5 | 24 | 1000.5 |
