This project involved the design and development of 3D regional biogeochemical models. The biogeochemical models were designed for attachment to the Harvard Hierarchy of models, including the surface boundary layer (SBL) component of the open ocean baroclinic eddy resolving quasigeostrophic (QG) model, or its coastal extensions, and the open boundary primitive equation (PE) model with hybrid vertical coordinates. Selection of these physical models is motivated by the interest to study the time evolution of biogeochemical components with realistic physical fields, as they all have data assimilation schemes associated. The approach taken in this model design was first to construct simplified models with a minimal number of biogeochemical components, and then step-by-step to introduce further components and detailed intercomponent interactions. These simplifications allow focused research on important biological mechanisms in the presence of realistic physical fields. Construction of regional biogeochemical models with data assimilation schemes will provide a unique tool to determine processes and sensitivities operative in the real ocean and to facilitate efficient experimental design and model validation. The simplest biogeochemical model of interest is of the nitrogen cycle. During this contract we constructed two models with two different compartmentalizations of the nitrogen components.