A dynamic mechanistic mathematical model (FeLANIM) is described, capable of simulating the major documented interactions between iron, light, ammonium, and nitrate in phytoplankton. There are various points in the model where species- or group-specific (e.g. eukaryote vs. prokaryote) details may be altered. Cellular Fe-containing processes accounted for in the model are photosynthesis, oxidative phosphorylation, and nitrate assimilation; synthesis of photosystems and nitrate and nitrite reductase (NNiR) are functions of the surplus Fe quota (i.e. total Fe quota minus Fe in functional components). Model simulations were run using a range of different physiological parameters for Fe-dependent processes and contrasting ammonium–nitrate interaction scenarios. Model output indicated the following interactions. Because the proportion of Fe in photosystems varies with irradiance, it is not possible to have a single Fe cost for nitrate assimilation. Fe stress can affect the relationship between ammonium and nitrate assimilations (the f-ratio), with a more rapid depression of nitrate use at lower concentrations of ammonium. However, depending on the degree of the repression of nitrate assimilation at high N:C, such a result may not be universal, and further experimental studies are required to clarify this issue. Fe refeeding results in a rapid recovery of growth rate accompanied by a proportionately greater increase in the amount of chl a within 24– 48 h. Estimates of enhanced production in iron fertilization experiments based only on increases in pigment may thus be exaggerated. Stimulation of NNiR activity on Fe refeeding appears indirect, through enhancement of photosynthesis rather than relief of nitrate stress. During Fe refeeding at high light, N nutrient transport increases proportionately more in nitrate-fed cells than in ammonium-fed cells. Changing the N source supplied to simulated Fe-stressed cells from nitrate to ammonium results in a rapid increase in growth rate and iron use efficiency with an increase in pigment content as NNiR content declines. When nitrate replaces ammonium, acclimation is slower because of the redirection of Fe formerly associated in photosystems to NNiR. Manipulation of the model may prove useful as an indicator for new research, by revealing elements of physiology that may be most significantin determining competitive advantage between species.