Three-component (NPZ), four-component (NPZD), and five-component (NNPZD) nitrogen-based ecosystem models are compared. The fixed points of the zero-dimensional systems, with no spatial variation except light attenuation by water, are determined. A linear-stability analysis shows that unstable steady solutions exist for all three models. Time-periodic solutions are found in these regions. It is shown that the choice of the values of the parameters in the zooplankton equation is critical in determining steady state concentrations. One-dimensional model studies allowing variation with depth, self-shading by phytoplankton, and vertical diffusion show that the differences among the models increase when the available light is influenced by the presence of phytoplankton. With vertical diffusion, periodic solutions, such as those in the zero-dimensional case, are not found. Finally, a set of one-dimensional depth-integrated models, with variability in time and across-shore coordinate, are formulated. These models include horizontal advection and diffusion and the sinking of detritus (or phytoplankton). Application of a constant offshore advection velocity and coastal boundary conditions consistent with upwelling, produces many of the features seen in two-dimensional experiments <Spitz et al., 2003>. The dependence of the resulting spatial distributions of phytoplankton and zooplankton on the values of the biological parameters is determined, helping to explain the two-dimensional results. Significant differences are found between the NPZ and the other models as a result of the sinking of phytoplankton rather than detritus.