Grazing control of phytoplankton stock in the open subarctic Pacific Ocean: a model assessing the role of mesozooplankton, particularly the large calanoid copepods Neocalanus spp.

Abstract

An ecosystem process model is developed and used to probe the hypothesis that grazing by zooplankton controls the standing stock of phytoplankton at Station P (50°N, 145'W) in the eastern subarctic Pacific Ocean The model attempts to reproduce seasonal variahons and integrated interactions of specific physical, chemical, and biological components of the subarctic pelagic ecosystem, and to simulate quantitatively the observed level of phytoplankton production. In its various versions the model describes the annual cycle of standing stock and production of phytoplankton, herbivorous microzooplankton, and the concentration of dissolved inorganic nitrogenous nutrients in a homogeneous mixed layer. Mesozooplankton (which include the large suspension-feeding copepods Neocalanus spp.) are assessed for their potential a s dominant grazers of phytoplankton and, alternatively, a s omnivorous consumers of phytoplankton and herbivorous rnicrozooplankton. Thls model indicates that a hypothesized trophic structure, in w h c h herbivorous rnicrozooplankton are the major grazers of phytoplankton and mesozooplankton are indmriminate omnivores, is plausible in theory and consistent with available but limited observations.