Future Changes in Biological Activity in the North Pacific Due to Anthropogenic Forcing of the Physical Environment

Many studies have examined the physical changes expected in the environment as a result of anthropogenic forcing. These physical changes will have an effect on ecosystems as well. In this study, a nitrogen-phytoplankton-zooplankton (NPZ) model is used to examine the effects of changes in the physical environment on primary productivity in the North Pacific ocean. The physical variables considered are mixed layer temperature and depth, solar insolation, and large-scale upwelling. The changes in these fields by the 2090s are taken from a coupled ocean-atmosphere general circulation model forced by projected atmospheric CO2 and sulfates, then applied to the NPZ biological model. The result is a change in the seasonal cycle of phytoplankton and herbivore concentrations across the subpolar North Pacific, moving from a regime characterized by strong variability with low wintertime values and a spring bloom, to much more constant yearly values. A reduction of yearly-averaged primary productivity accompanies much of this shift to more constant year-round conditions. In other regions, productivity increases as warmer surface waters enable higher growth rates. Changes in mixed layer temperature and depth account for almost all the changes in productivity; model-predicted changes in surface insolation and large-scale upwelling have little impact.