The population dynamics of Pseudocalanus elongatus have been investigated within the framework of the GLOBEC-Germany project to gain a better understanding of its life cycle and population dynamics and to estimate secondary production in the North Sea. During an intensive field study in the German Bight between February and October 2004, experiments on reproduction were performed and data on length of copepodids and abundance were collected to characterize the population in the southern North Sea. This data set was used to update the literature- based parameterization of a population model for P. elongatus to investigate the population dynamics, life history and production in the German Bight. The ability of data to improve population models is also discussed. Pseudocalanus elongatus was found to be a major contributor to carbon uptake contributing about onethird of copepod production. Though the spatial variability in field observations was not reflected by the model, the simulation matched data within one order of magnitude at most stations. The high-resolution field observations and experiments mainly improved the parameterization of the reproductive parameters. Mortality is found to be a critical parameter due to its influence on population size. Using constant rates, though based on observation-derived estimates, seems not to capture realistic variability. Our study confirms the need for experimental and field data to build a robust parameterization for concentration-based population models.