A MODELING STUDY OF PROCESSES REGULATING PLANKTON STANDING STOCK AND PRODUCTION IN THE OPEN SUB-ARCTIC PACIFIC-OCEAN

At Ocean Station P (50-degrees-N 145-degrees-W) in the open subarctic Pacific Ocean recent estimates of phytoplankton production rate were consistently much higher than those obtained during previous decades of weathership operations. Moreover, the production was supported to a major extent by regenerated forms of nitrogen despite high concentrations of NO3. An ecosystem process model, simulating the interactions among phytoplankton, herbivorous microzooplankton and nutrients, was used to evaluate the possible mechanisms underlying both the high production rates and the patterns of utilization of dissolved nitrogenous nutrients. The model results suggest that at Station P the phytoplankton have specific growth rates expected for the ambient light and temperature regime, and furthermore that two processes, grazing control of phytoplankton stock and preferential utilization of NH4 by the phytoplankton, can account for the continuously low standing stock of phytoplankton and the underutilization of NO3 in the surface layer in summer. However, moderate limitation of phytoplankton specific growth rate by a trace nutrient such as dissolved iron cannot be ruled out. Winter observations of plankton dynamics at Station P would afford a strong test of the model predictions. In addition, the model highlights several properties of the subarctic Pacific epipelagic ecosystem in need of further investigation. These include possible trace nutrient limitation of maximum specific growth rate of the phytoplankton assemblage; the abundance, types and physiological characteristics of microzooplankton grazers; the sources and intensity of predatory mortality of grazers; effects of daily variability of physical processes on plankton dynamics; and control of growth rates of individual phytoplankton species, especially microphytoplankters whose specific growth rates may be limited by availability of trace nutrients.