A simple Atlantic ecosystem model coupled to MICOM is compared with a similarly configured HYCOM model. Differences point to the sensitivity of the ecosystem models to small changes in physical interactions between mixed layer and thermocline. While there are few mixed layer measurements for comparison with model fields, satellite observations of ocean color provide high spatial and temporal coverage observations of parameters that depend on the critical interactions between the mixed layer and thermocline that regulate not only surface biomass and production, but also the ventilation of the thermocline and uptake of atmospheric CO2.