We define a new dimensionless number, S, to be the ratio of nitrogen supply to nitrogen demand of new primary production in the pelagic ecosystem. When S > 1, we expect high-nutrient, low-chlorophyll (HNLC) conditions. Using the results of a new model of nitrogen input and consumption for the mixed layer of the ocean, we calculate S for selected oceanic regimes. Those generally accepted to be HNLC are characterised by S > 1. The bio-optical terms in this model (specific absorption of pigments, parameters of the light-saturation curve) are known to respond to addition of iron. Using these known responses, we recalculated the expected value of S under hypothetical enrichments of the selected regimes with iron. In each case, the magnitude of S was reduced, but not always below unity. The maximum value of chlorophyll biomass that can be sustained in a given mixed layer may be calculated from consideration of either the bio-optics or the nitrogen supply. The maximum realised biomass will be the smaller of these 2 estimates.