The phytoplankton macro nutrients nitrate, nitrite, silicate, phosphate and ammonium are analyzed in seawater using a colorimetric assay in which light absorbance is measured versus known standards. To analyze for nutrients a Seal Analytical continuous-flow AutoAnalyzer 3 (AA3) is used. After each run, the charts are reviewed for any problems, any blank is subtracted, and final concentrations (micro moles/liter) are calculated.
Complete methods for nutrient determination can be found here.
Silicate is analyzed using the technique of Armstrong (1967). An acidic solution of ammonium molybdate is added to a seawater sample to produce silicomolybdic acid which is then reduced to silicomolybdous acid (a blue compound) following the addition of stannous chloride. Tartaric acid is added to impede PO4 color interference. The sample is passed through a 10mm flow cell and the absorbance measured at 660nm.
A modification of the Armstrong (1967) procedure is used for the analysis of nitrate plus nitrite. For this analysis, the seawater sample is passed through a cadmium reduction column where nitrate is quantitatively reduced to nitrite. Sulfanilamide is introduced to the sample stream followed by N-(1-naphthyl) ethylenediamine dihydrochloride which couples to form a red azo dye. The stream is then passed through a 10 mm flowcell and the absorbance measured at 520nm. The same technique is employed for nitrite analysis, except the cadmium column is not present. Nitrate concentration is calculated by subtracting the nitrite value from the combined Nitrate + Nitrite (N+N) value.
Phosphate is analyzed using a modification of the Bernhardt and Wilhelms technique. An acidic solution of ammonium molybdate is added to the sample to produce phosphomolybdic acid, then reduced to phosphomolybdous acid (a blue compound) following the addition of dihydrazine sulfate. The reaction product is heated to ~55C to enhance color development and then passed through a 10 mm flow cell and the absorbance measured at 820 nm.
Ammonium is analyzed via the Berthelot reaction in which hypochlorous acid and phenol react with ammonium in an alkaline solution to form indophenol blue. The sample is passed through a 10 mm flow cell and measured at 660nm. This method is a modification of the procedure by Koroleff (1969,1970).
An aliquot from a large volume of stable deep seawater is run with each set of samples as an additional check. The stability of the deep seawater check is aided by the addition of mercuric chloride as a poison.
The efficiency of the cadmium column used for nitrate reduction is monitored throughout the cruise and usually ranges from 98.0-100.0%.