The phytoplankton macro nutrient Ammonium is analyzed in seawater using a colorimetric assay in which light absorbance is measured versus known standards.


1. Ammonium Overview

California Cooperative Oceanic Fisheries Investigations (CalCOFI) added Ammonium to their nutrient sampling dataset (Nitrate, Nitrite, Silicate and Phosphate) in November 2004. Seawater samples are collected for 66 CalCOFI and 9 SCCOOS stations located in the California Current Ecosystem (CCE). Each sample depth is collected in a clean (10% HCl rinsed) 40 ml polypropylene centrifuge tube and analyzed within 2-16 hours on a Seal Analytical continuous-flow AutoAnalyzer3. 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 66 nm. This method is a modification of the procedure by Koroleff (1969, 1970). Data is reported in micro-moles/liter. Ammonium is a difficult parameter to measure accurately due to its' insidious nature and problems with contamination.

A detailed method description for the macro nutrient Ammonium can be found on the CalCOFI website at

2. References

  • Gordon, L.I., J.C. Jennigs, A.A. Ross, and J.M. Krest. 1993. A Suggested Protocol for Continuous Flow Automated Analysis of Seawater Nutrients (Phosphate, Nitrate, Nitrite and Silicic Acid) in the WOCE Hydrographic Program and the Joint Global Ocean Fluxes Study. WOCE Hydrographic Program Office, Methods Manual WHPO 91-1.