This study relates the performance of an optimized one-dimensionalecosystem model to observations at three sites in the North Atlantic Ocean:the Bermuda Atlantic Time series Study (BATS, 32N 64W),the location of the North Atlantic Bloom Experiment (NABE, 47N 20W),and Ocean Weather Ship INDIA (OWS-INDIA, 59N 19W).The ecosystem model resolves dissolved inorganic nitrogen (N),phytoplankton (P), zooplankton (Z) and detritus (D), therefore calledNPZD-model. Physical forcing, such as temperature, eddy diffusivitiesand surface radiation are taken from an eddy-permitting generalcirculation model of the North Atlantic Ocean, covering a period from1989 through 1993. When an optimized parameter set is applied, theNPZD-model produces substantial differences in the biogeochemicalfluxes with respect to model results using previously published``traditional'' parameter values.Annual nitrogen fluxes are determined forthe upper 126 meters together with standard deviations which areassociated with uncertainties in the optimal parameter estimates. Incontrast to earlier models the rapid cycling of organic matter forsustaining primary production is significantly enhanced. An analysis ofprimary production reveals systematic discrepancies between$^{14}$C-fixation rates and modelled primary production under nutrientdepleted conditions.The chosen physical boundary conditions are adequate to simulate thebiogeochemical fluxes at the BATS and NABE sites. At high latitudes(OWS-INDIA), however, the physical-biological interactions in themodel cannot represent the observed chlorophyll distribution in spring.During this period the short-termed alterations of stratification andmixing together with a rapidly following biological responseare insufficiently resolved in order to reproduce chlorophyll concentrationsfound at depths of 150-200m.