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AMT4 Nutrient (micromolar) measurements from CTD bottle samples

Originator's Protocol for Data Acquisition and Analysis

This data originates from analyses of bottle samples taken from 35 CTD casts.

The nutrient samples were generally collected from the second cast of the day. The concentrations of nutrients were measured colourimetrically using a 4-channel, Technicon auto-analyser with standard methodologies. All depths from the CTD (typically 10) were measured on each cast, generally within 2 hours of collection. Surface samples were not taken from the pumped supply. A total of 35 casts were analysed, 7 of the casts were to depths of 1000m on this voyage, whereas previously maximum depths achieved were of the order 200m.

All samples were analysed on board within hours of collection and the data processed to calibrated and salinity corrected digital values. The overall patterns were very similar to previous AMT transects. Total nitrogen was only detectable in the surface waters south of Montevideo and north of 47oN. Through the tropics and between these positions, total nitrogen was below detection in the surface waters but could be measured in samples from below the thermocline. Although the value of macro-nutrients measured in surface waters are of limited value to biogeochemical studies when the values fall below detection, the data from depth is still useful in the interpretation of primary production. The last four AMT cruises have all been analysed using the same instrument and identical standardisation procedures. Maximum values for all three nutrients were not large during AMT1 because sampling depths tended to be shallower then. However, a plot of the ratio of total nitrogen (µmol N) to total P (µmol P) obtained on each survey show that all four previous cruises returned similar ranges of values and that the ratios were reasonably consistent. AMT4 data fell within an envelope of all the previous AMT nutrient data when plotted in this way, which provides a measure of confidence in the data.

References Cited

Instrumentation Description

Not relevant to this data set.

BODC Data Processing Procedures

Data were submitted to BODC in Microsoft Excel spreadsheet format and saved to the BODC archive with reference PML010014. Sample metadata were checked against information held in the database. Originator's sample ID was matched based on latitude/longitude and bottle firing depth. Station/Sample reference provided is the Julian Day (JD) and the cast that sampling took place on. These were matched to the CTD casts that took place each day on the cruise.

OIDs were present in the database for the CTD events in the format CTDPxxx, where xxx was the cast number for the cruise. CTDP049 (137c) had four depths in the originator's file (700, 800, 900 and 1000 m) that were not listed in the cruise report as depths at which bottles were fired. The data was loaded linked to the depths in the cruise report (800, 1000, 1500 and 2000 m).

There were replicate samples provided for one depth on a number of casts. The mean of the replicates were taken for each depth and the values entered into the database for the following samples.

There was an unlabelled cast of data in the originator's file, which could not be loaded to the database. The nitrite sample at 175 m from CTD014 (122a) was below the detection limit but the detection limit has not been specified.

Parameter codes defined in BODC parameter dictionary were assigned to the variables. 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;

Data Quality Report

BODC has not been advised of any quality checks carried out by the data originator. BODC has highlighted negative values with the BODC 'M' flag for suspect data.

Problem Report

Not relevant to this data set.