Nitrogen fixation vertical profiles, diel surveys and variation in bioassay experiments using 15N-dinitrogen tracer incubations, UK SOLAS cruise D325.

Originator's Protocol for Data Acquisition and Analysis

Rates of nitrogen fixation were measured as part of a series of rate and state measurements of nitrogen cycling and microbial productivity carried out at each of the six oceanographic stations occupied during the UK SOLAS D325 cruise. Measurements were made under a semi-lagrangian framework and involved at each site:

a) A vertical profile between the surface and base of euphotic zone;

b) A diel survey over 24 hours from surface waters;

c) A bioassay experiment to assess limitation of microbial processes by essential trace metals (Fe, Zn, Co, Cu) and nitrogen nutrients.

Vertical profiles were performed using water collected from the titanium CTD rosette in pre-cleaned (10% HCl), 10 litre "Niskin" bottles. Diel surveys and bioassays were performed using seawater collected from a trace metal clean towed fish deployed from the port aft quarter.

Bioassay experiments were performed under a strict trace metal clean regime and involved the collection of 120 litres seawater from the depth equivalent to 55% of surface PAR, into an acid clean HDPE container, which was housed in the trace metal clean container. This was then distributed into series of triplicate 4.6 litre bottles to be used as controls or submitted to nutrient additions including iron, zinc, cobalt, copper, nitrogen nutrients (NH4+NO3) or dust samples. These bottles were placed in on-deck incubators at surface temperature and 55% incident irradiation for a conditioning period of 48 hours, after which nitrogen fixation rate was determined.

For nitrogen fixation measurements seawater was distributed into triplicate 1-litre polycarbonate bottles and amended with 2 ml of ¹⁵N-N₂. Following incubation in the on-deck incubators for approx 6 hours, experiments were terminated by filtration onto 25 mm GF/F filters. The filters were dried onboard and pelleted into tin capsules. Particulate nitrogen and¹⁵N atom% were measured using continuous-flow stable isotope mass-spectrometry (PDZ-Europa 20-20 and GSL; Owens and Rees, 1989) and rates determined according to Montoya et al., (1996).

References Cited

Montoya, J.P., Voss, M., Kahler, P., and Capone, D.G. (1996) A simple, high-precision, high-sensitivity tracer assay for N-2 fixation. Appl. Environ. Microbiol. 62: 986-993.

Owens, N.J.P., and Rees, A.P. (1989) Determination of N-15 at Submicrogram Levels of Nitrogen Using Automated Continuous-Flow Isotope Ratio Mass-Spectrometry. Analyst 114: 1655-1657.

Instrumentation Description

PDZ-Europa 20-20

BODC Data Processing Procedures

Data from CTD profiles, diel surveys and bioassay experiments were submitted in three separate Excel spreadsheets and saved into the BODC archive system.

The vertical profile data were submitted with the following metadata field headers: date, station, CTD cast number, CTD bottle reference, depth of sample collection, and percent of surface light. The records were merged into BODC's database by matching the CTD cast number and the CTD bottle reference to existing records. The metadata were then cross-checked and major discrepancies investigated. A major depths mismatch was observed at station F110 and sourced to a change in bottle firing routine. The percent of surface light depth provided by the originators was used to ascertain that the measurements were linked to the correct sample metadata in the database.

Data from the diel surveys were submitted with the following metadata field headers: date, station, time, latitude, and longitude. Time was confirmed as UTC. The following amendments were made by BODC prior to ingesting the data into its database: 1) Originator's sampling event identifiers were generated by BODC based on information submitted and information held in the cruise report; 2) a period of 24 hour was added to the sampling time of the last sample of each diel survey in order to correct for a consistent clerical error in the reported date.

Data from the bioassay experiments were submitted with the following metadata field headers: date, station, sample time, (incubation) time, and treatment. Values in these fields and information provided in the cruise report were used to populate BODC's incubation metadata schema. Time was confirmed as UTC.

In all three cases and as advised by the data originator, the start time of the incubation was set to 1.5 hours after sampling time, this being either the end of the CTD cast or the sampling time provided by the originator.

The variables were assigned parameter codes defined in BODC parameter dictionary and data were loaded into BODC's database using established BODC data banking procedures.

A parameter mapping table is provided below;

Data Quality Report

BODC were not advised of data quality issues by the data originator.

Problem Report

Not relevant to this data set.

Metadata

Bottle Flags