Simulations of Marine Ecosystem Response to Climate Variation with a One Dimensional Coupled Ecosystem/Mixed Layer Model

Existing observations are inadequate to identify and to understand the processes by which oceanic and atmospheric variability affect the marine biota at climate scales. To aid in the identification and study of important processes, we employ a one-dimensional coupled mixed layer / planktonic ecosystem (Nutrient-Phytoplankton-Zooplankton-Detritus) model of the subarctic Pacific Ocean. Increasing evidence indicates that the subarctic Pacific, along with the Southern Ocean and the equatorial Pacific, are High Nutrient-Low Chlorophyll (HNLC) regimes because of the scarcity of the micronutrient iron. Under the assumption that the availability of iron might vary as the climate varies, we test the response of our model to a simple formulation of iron limitation. Simulations with and without iron limitation of primary production by phytoplankton yield a difference of about a factor of two in the standing stock of zooplankton during the productive season, although changes in phytoplankton biomass are small. A similar difference in zooplankton biomass has been observed over a 30 year period in the eastern subarctic Pacific (Brodeur and Ware, 1992). The modelled result occurs whether we employ a linear or a quadratic formulation for zooplankton losses. An important implication of this result is that a doubling in zooplankton biomass would mean more food for fish populations, yet contemporary plans to monitor ocean productivity from satellites would detect only minor changes in phytoplankton biomass.