A one-dimensional coupled physical bio-geochemical model of the surface ocean was constrained to reproduce the seasonal evolution of surface chlorophyll, temperature and nitrate concentration by adjusting the parameters of a simple trophic system using a variational technique. It is found that assimilation of surface chlorophyll data only not allow a robust constraint of the simulated trophic cycle. When surface chlorophyll, temperature and nitrate data are used simultaneously, seven linear combinations of the model parameters can be adjusted. While temperature values are useful to constrain vertical diffusion below the mixed layer, the seasonal nitrate evolution (2 values/year) is a strong constraint on the balance between vertical diffusion and particle sinking rate. The three types of surface information provide robust constraints on the model parameters and on the main carbon fluxes within the euphotic layer. For the model to reproduce observed surface pCO2 concentrations, it is found as necessary to adjust the C/N ratios. An example is given, where the C/N ratio for net organic matter export from euphotic zone is increased to 12¿1 and the C/N ratio for detritus remineralization is decreased to 3.3¿2, rather than the canonical value of 7. ¿ American Geophysical Union 1996