A comparison of two lower trophic models for the California Current System

Both simple and more complex lower trophic ecosystem models were applied to the near shore pelagic zone of the California Current System. The ecosystem models were a simple nitrogen–phytoplankton–zooplankton (NPZ) model derived from that of Edwards et al. (J. Mar. Res. 58, 37–60, 2000), and a modified version of the more complex PICES North Pacific Ecosystem Model for Understanding Regional Oceanography (NEMURO), which includes both nitrogen and silicate cycles, as well as multiple phytoplankton and zooplankton components. These models were framed within two simplified physics models, similar to those constructed by Newberger et al. (J. Geophys. Res. 108 (3), 3061, doi:10.1029/2001JC001182, 2003). The first physics model was a zero-dimensional closed system, used for structural analysis of the biological models. The second was a one-dimensional (horizontal, with no vertical structure) cross-shelf model of the mixed layer, used to examine the biological response to upwelling. The predictions of the two biological models within the one-dimensional physical framework were compared with observed nutrients, chlorophyll, and zooplankton on the Newport Hydrographic Line. Equilibrium behavior of the zero-dimensional models was similar to that in previous studies; differences between the models resulted primarily from differences in the density dependence of mortality. Both models were found to be highly sensitive to the same four parameters: source nitrate concentration, maximum grazing rate, zooplankton Ivlev constant, and zooplankton mortality rate. In the cross-shelf case, both biological models had a similar timing of phytoplankton response to upwelling, but maximum phytoplankton concentration and zooplankton dynamics differed, with the simpler NPZ model resulting in lower dissolved nutrients but higher plankton densities than the modified NEMURO model. While the modified NEMURO model provided a slightly better overall fit to observations, neither model was able to fully capture observed phytoplankton and zooplankton dynamics.