Abstract

For many years the concentrations of inorganic nitrogen and phosphorus that limit the growth of marine phytoplankton were below the limit of detection by traditional colorimetric methods of analysis. In the case of nitrate, that situation changed dramatically with the development of the chemiluminescent assay for nitrate. Concentrations of nitrate measured with this technique in the surface waters of some parts of the open ocean have been found to be consistently less than 5 nM. Although analysis of phosphate is still based on colorimetric methods, the availability of a convenient radioactive tracer, either ³²P or ³³P, has made it possible to determine the relationship between phosphate concentrations and growth rates of laboratory cultures and to test the accuracy of the presumed hyperbolic relationship (Monod equation) between growth rate and substrate concentration. Similar studies in the case of ammonium-limited growth continue to be problematic because there is no convenient radioactive tracer for nitrogen and there is no assay for ammonium analogous to the chemiluminescent assay for nitrate. All available evidence indicates that limiting inorganic nitrogen and phosphate concentrations in the ocean are on the order of nanomolar to perhaps a few hundred nanomolar.