Weak constraint parameter estimation is applied to tune a local4-component (phytoplankton, zooplankton, detritus, dissolved inorganicnitrogen) model for the ecosystem of the upper mixed layer in the NorthAtlantic. The model is constrained by the mean monthly Nimbus-7chlorophyll data. The basin under consideration is separated into5 x 5 degree boxes where the model is optimizedindependently. Adjusted model parameters exhibit significant spatialvariations. We show how to evaluate a posteriori the relativecredibility of the data and the model. A method for estimatingthe accuracy of model parameterizations is suggested.For Bermuda Station "S", the major contribution to model errors comesfrom uncertainties in parameterizations for the phytoplankton mortality,zooplankton losses and breakdown of detritus.