Dissolved inorganic nitrate, nitrite, silicate and phosphate concentrations of seawater sampled during the Antarctic Circumnavigation Expedition (ACE) during the Austral Summer of 2016/2017.

***** Dataset abstract *****

This dataset contains dissolved inorganic nitrate, nitrite, silicate and phosphate concentrations of seawater sampled during the Antarctic Circumnavigation Expedition (ACE) Legs 1-3. Water samples were collected from the underway seawater supply every 3 hours, preserved and analysed for dissolved inorganic nutrient concentrations using flow injection and colorimetric methods. These samples provide an estimate of the dissolved concentrations of inorganic macronutrients essential for phytoplankton growth.

***** Original data collection *****

Seawater samples for the analysis of dissolved inorganic nutrients were collected from the underway seawater supply every 3 hours. Seawater was collected in individual 15 millilitre or 50 millilitre polypropylene tubes at each time point. Sample tubes were rinsed with seawater 2-3 times before being filled and were stored upright at -20 degrees Celsius until analysis post voyage in October 2017.

The seawater samples were transported frozen to the University of Cape Town for analysis at the Marine Biogeochemistry Laboratory, Department of Oceanography, University of Cape Town. Flow injection and colorimetric analyses were performed using a Lachat QuickChem Flow Analyzer. Spectrophotometry was performed on a Thermo Scientific Geneysis 30 Visible spectrophotometer.

Nitrate concentrations ([NO3-]; micromolar) were measured and determined following the methods of Stanley et al. (1994), Wendt (2000) and Grasshoff et al. (1999) by first measuring total oxidised nitrogen (NOx=[NO3-]+[NO2-]; micromolar) and subtracting nitrite ([NO2-]; micromolar), analysed separately, to yield nitrate concentrations [NO3-]. Each sample was passed through a copperized cadmium column, which reduced the sample [NO3-] to [NO2-] aided by the addition of ethylenediaminetetracetic acid (EDTA; Wendt, 2000). Reagents sulphanilamide (0.2 millilitres; 0.2 N) and N-(1-napthyl)-ethylenediamine dihydrochloride (N-NED; 0.004 N) were added in-line (Strickland and Parsons 1972) and the final concentration of NOx was determined spectrophotometrically by measuring absorbance at 520 nm (Wendt, 2000). Standards of 40 micromolar, 20 micromolar, 8 micromolar, 4 micromolar, 2 micromolar and 1 micromolar [NO3-] were prepared from a gravimetric stock (10 millimolar) and analyzed daily, yielding a standard curve covering the concentrations expected for each run of 100 samples. Standards of 20 micromolar and 8 micromolar were also run after every 10 samples to monitor instrument performance and allow instrument drift to be corrected.

Nitrite concentrations ([NO2-]; micromolar) were analysed following the colorimetric method of Bendschneider and Robinson (1952). After the addition of sulphanilamide (0.1 millilitres, 0.05 N) and N-NED (0.1 millilitres, 0.004 N) reagents to 5 millilitres of sample, the absorbance at 543 nm was measured using a spectrophotometer. A daily standard curve of [NO2-] was prepared from a gravimetric stock (10 millimolar) at the following concentrations: 5 micromolar, 2.5 micromolar, 1.25 micromolar, 0.65 micromolar, 0.3 micromolar, and 0.1 micromolar.

Silicate concentrations ([Si(OH)4]; micromolar) were measured using the colorimetric analysis of Grasshoff et al. (1999) and Koroleff (1971). The reagents ammonium molybdate (0.2 millilitres; 0.2 N), ascorbic acid (0.2 millilitres; 0.2 N) and oxalic acid (0.2 millilitres; 1 N) were added to the sample before absorbance was measured at 810 nm. Standards of 100 micromolar, 75 micromolar, 50 micromolar, 20 micromolar, 10 micromolar, 5 micromolar and 2.5 micromolar of [Si(OH)4] were prepared from a gravimetric stock (10 millimolar) and analyzed daily, yielding a standard curve covering the concentrations expected for each run of 100 samples. Standards of 20 micromolar and 10 micromolar were also run after every 10 samples to monitor instrument performance and allow instrument drift to be corrected.

The colorimetric method of Strickland and Parsons (1972) was followed for the determination of inorganic phosphate concentration ([PO43-]; micromolar). The reagents ammonium paramolybdate (0.5 millilitres; 0.15 N), sulphuric acid (0.5 millilitres; 8 N), potassium antimonyl-tartrate (0.5 millilitres; 0.002 N) and ascorbic acid (0.5 millilitres; 0.3 N) were added to the sample before absorbance was measured at 880 nm. A daily standard curve of ([PO43-]) was prepared using the following standards made up from a gravimetric stock (10 millimolar): 5 micromolar, 2.5 micromolar, 1.25 micromolar, 0.65 micromolar, 0.3 micromolar, and 0.1 micromolar. 

The detection limits for the nutrient analyses were 0.1 micromolar for total oxidised nitrogen, nitrate and silicate, and 0.05 micromolar for nitrite and phosphate.

Information on the underway seawater supply can be found in the ACE Cruise Report (Walton and Thomas 2018, DOI: 10.5281/zenodo.1443511).

***** Data processing *****

Latitude and longitude for each datapoint have been extracted from the ACE GPS one-minute resolution dataset (Thomas and Pina Estany 2019; DOI: 10.5281/zenodo.3483166), matched by date and time.

***** Quality checking *****

Following flow injection and colorimetric analyses, sample data were flagged if the derived nutrient concentration (micromolar) was below the limit of detection with an ‘LOD_Flag’ for each nutrient where 1 = ‘good’ and 2 = ‘below limit of detection’ (see data_file_header.txt).

***** Standards *****
 
This dataset does not conform to any specific standards.

***** Dataset contents *****

- README.txt, metadata, text
- data_file_header.txt, metadata, text
- ace_uw_nutrients_20200527CURRSGCMR.csv, data file, comma-separated values
- change_log.txt, metadata, text

***** Dataset contact *****

David Antoine, Curtin University, Australia. ORCID: 0000-0002-9082-2395. Email: david.antoine@curtin.edu.au
Charlotte Robinson, Curtin University, Australia. ORCID: 0000-0001-8519-5641. Email: charlotte.robinson@curtin.edu.au, charlotte.mary.robinson@gmail.com

***** Dataset citation ******

Please cite this dataset as:
Thomalla, S., Fawcett. S., Antoine, D., Berliner, D., Forrer, H., Little, H., Moutier, W., Olivier, A., Robinson, C., Ryan-Keogh, T. and Schuback, N. (2020). Dissolved inorganic nitrate, nitrite, silicate and phosphate concentrations of seawater sampled during the Antarctic Circumnavigation Expedition (ACE) during the Austral Summer of 2016/2017. (Version 1.1) [Data set]. Zenodo. https://doi.org/10.5281/zenodo.3903134