N uptake data obtained from 15N tracer techniques were surveyed in surface waters of the Subantarctic and Polar Frontal Zones southwest of Tasmania during summer 1998. The N flux rates and f ratio were computed using inverse modeling. The approach provides a general framework for the handling of random and systematic variations. Furthermore, factorial experiments based on controlled ammonium additions were used to study the sensitivity of the f ratio relative to perturbations of the regenerated nutrient supply. This was achieved in distinct regions: the Subtropical Convergence Zone, the Subantarctic Zone, the Subantarctic Front, and the Polar Frontal Zone. Overall, the stability of the f ratio appeared closely related to the availability of both dissolved iron and ammonium. The effect of one factor depended on the level of the other one, and the interaction influenced the f ratio patterns. The relationship between f ratio and ammonium supplementation ranged from linear to convex curves, with the steepest slopes in water masses rich in dissolved iron and with low ammonium content. The magnitude of this variation may have important ecological and geochemical consequences. Our results suggest that only versatile systems are able to exhibit events of high new production.