Weak constraint parameter estimation is applied to tune a 0-D 4-component (phytoplankton, zooplankton, detritus, dissolved inorganic nitrogen) model for the ecosystem of the upper mixed layer in the North Atlantic. The model is constrained by the monthly mean Nimbus-7 chlorophyll data. The basin under consideration is separated into 5 x 5° boxes where the model is optimized independently. Adjusted model parameters exhibit significant spatial variations. We also show how to evaluate a posteriori the relative credibility of the data and the model. A method for estimating the accuracy of model parameterizations is suggested. For Bermuda Station "S", the major contribution to model errors comes from uncertainties in parameterizations for the phytoplankton mortality, zooplankton losses and breakdown of detritus. © 2003 Elsevier B.V. All rights reserved.