NIWA Ocean Survey 20/20

Coastal and Marine Data Portal

New Zealand Ocean Acidification Observing Network (NZOA-ON)

Methods Used

Method Overview

  • NIWA provides sampling partners with a sampling crate containing bottles, preservative, chemically resistant gloves, calibrated thermistor and paperwork (sample record sheet, health and safety information, freighting labels).
  • Water samples are taken fortnightly at each site by the sampling partners.
  • The preserved water samples are returned to the laboratory at the NIWA /University of Otago Research Centre in Oceanography in Dunedin when the sample crates are full.
  • The samples are then analysed for DIC and alkalinity,
  • The salinity is measured, and the nutrient concentration is estimated using dipsticks.
  • The digital thermometers are recalibrated.
  • Various carbonate parameters are calculated from the measured data: pHT, pCO2, Ωaragonite, Ωcalcite and [CO32-].

 

Methods for Analysis and Calculations

DIC

The concentration of dissolved inorganic carbon in seawater (CT or more commonly called DIC) is defined as

DIC equation

 where the [ ] represent the concentrations of each of the species.

DIC is measured using coulometric analysis of the carbon dioxide evolved from the acidified sample, a method described by (Dickson et al., 2007). Briefly –an accurately known volume of the seawater sample is dispensed into a stripper containing phosphoric acid. The carbonate ions, bicarbonate ions and carbonic acid in the seawater react with the acid to form carbon dioxide gas. This gas is then purged with a carrier gas and swept into a reaction cell where the amount of carbon dioxide gas is determined by coulometry. DIC is reported in units of micromoles per kg of seawater (μmol kg-1).

A CRM (certified reference material, A Dickson, Scripps Institute of Oceanography) is analysed with each batch of samples, and the accuracy and precision of the method is estimated to be ± 1 μmol kg-1.

Alkalinity

The total alkalinity (AT) of a seawater sample has a very specific definition (Dickson et al., 2007), and can be summarised as:

 alkalinity equation

where the [ ] represent the concentrations of each of the species, and the … represents other minor acid or base species, some of which are important in coastal and anoxic environments.

Alkalinity is measured using a closed cell potentiometric titration, a method described by (Dickson et al., 2007). Briefly, an accurately known volume of the seawater sample is introduced into a reaction cell, which is completely filled and sealed. The initial emf (pH) of the sample is measured and then the sample is titrated with hydrochloric acid, the volume of HCl added and the emf (pH) are measured at each titration point. The resulting titration curve is analysed by comparing it with a theoretical curve using a least squares minimisation method. AT is reported in units of micromoles per kg of seawater (μmol kg-1).

A CRM (certified reference material, A Dickson, Scripps Institute of Oceanography) is analysed with each batch of samples, and the accuracy and precision of the method is estimated to be ± 2 μmol kg-1.

Salinity

Salinity is measured on the bottle sample using a YSI Pro30 Conductivity Instrument.

Temperature

Each sampling crate is provided with a xxx thermistor. The thermistor sensor is attached to a string with a float and a sinker, to ensure that the temperature is taken at the correct depth. The thermistors are calibrated before each sample crate is sent out, and again when they return. The water temperature that is recorded on the sampling sheet when the sample it taken is then corrected according to the calibration function.

Calculation Procedures

From the two measured values, DIC and AT, and the temperature and salinity of the samples, the other carbonate parameters can be calculated. We calculate pHT, pCO2, Ωaragonite and Ωcalcite. The calculations are done using the programme SWCO2, (http://neon.otago.ac.nz/research/kah/software/software.htm) and the recalculated Mehrbach constants (Dickson and Millero, 1987).

pHT is the pH on the total hydrogen ion concentration pH scale, reported at the in situ water temperature.

pH equation

where [H+] is the concentration of the free hydrogen ions plus the contribution from the total sulfate dissociation.
(Dickson et al., 2007) gives a good definition and description of the various pH scales.

pCO2 is the partial pressure of carbon dioxide in an air sample that is in equilibrium with the seawater sample, and is reported in units of microatmospheres (μatm).

Ωaragonite is the saturation state of the calcium carbonate mineral aragonite, and is related to the solubility of aragonite:

aragonite equation

Where K’sp is the solubility product constant for the dissolution of aragonite, and the [ ] represent the concentrations of each of the species.

Ωcalcite is the saturation state of the calcium carbonate mineral calcite, and is related to the solubility of calcite:

calcite equation

Where K’sp is the solubility product constant for the dissolution of calcite, and the [ ] represent the concentrations of each of the species.

Carbonate Ion Concentration, [CO32-], is reported in units of μmol kg-1

 

SeaFET™ Ocean pH Sensors

SeaFET™ pH Sensors (http://satlantic.com/seafet) are deployed for 4-5 months at each location on a roving basis. We are planning to have one pH sensor for each site, but currently the instruments are shared between the sites.

The SeaFET™ pH Sensors measure pH every 30 minutes, and the data is downloaded on retrieval.

References

Dickson, A.G. and Millero, F.J., 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research, 34: 1733-1743.

Dickson, A.G., Sabine, C.L. and Christian, J.R. (Editors), 2007. Guide to best practices for ocean CO2 measurements. PICES Special Publication 3. IOCCP Report No. 8, 191 pp. PICES Special Publication 3, 191 pp.