Ecosystem response to upwelling off the Oregon coast: Behavior of three nitrogen-based models

Abstract

The behavior of three ecosystem models is analyzed for upwelling off the Oregon coast as a function of the number of model components. The first ecosystem model includes dissolved inorganic nitrogen-phytoplankton-zooplankton (NPZ), the second (NPZD) adds detritus, and the third (NNPZD) splits the nutrients into nitrate and ammonium. The models are made as equivalent as possible by choosing the same parameterizations for the biological interactions and are coupled to a two-dimensional physical circulation model. Simulations with wind forcing from summer 1973 lead to similar spatial and temporal patterns for the three models. The mean phytoplankton concentration is maximum onshore of the upwelling jet while the mean zooplankton concentration is maximum farther offshore. The main difference among the models is a larger maximum zooplankton concentration for the NPZ simulation. This difference is attributed to the sinking of phytoplankton rather than detritus. We repeat the NNPZD summer 1973 simulation with wind forcing from 1999 and with constant wind stress. We also use the 1973 wind forcing and vary model parameters. The zooplankton maximum grazing rate is increased, the detritus sinking rate is reduced, and the inhibition of nitrate uptake by ammonium is removed. In all cases, the maximum of the zooplankton concentration is found offshore of the phytoplankton maximum. The locations and magnitudes depend upon the nature of the time variability of the wind forcing and on the biological parameters. The wind variability and the parameter values have little effect on the mean primary productivity, but have a larger effect on the f-ratio.