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.

The importance of zooplankton‐protozoan trophic couplings in Lake Michigan

“…Some field studies showed that a natural assemblage of microzooplankton (ciliates and zooflagellates) comprised a large portion of the C ingested by estuarine copepods (Gifford and Dagg 1988) heterotrophic flagellates could be important food for copepods, particularly at lower phytoplankton densities (Roman et al 1988). In addition, a model of grazing control of phytoplankton in the open subarctic Pacific Ocean could be balanced only by assuming that macrozooplankton are omnivorous and graze on both phytoplankton and protozoa (Frost 1987). Although our data do not demonstrate macrozooplankton selection for protozoa over phytoplankton, they suggest that protozoa are significant prey items in addition to phytoplankton in Lake Michigan.…”

von Liebig's law of the minimum and plankton ecology (1899–1991)

“…The importance of losses resulting from grazing cannot be underestimated (FRosT, 1987;BANSE, 1992;FROST and FRANZEN, 1992), especially where size selection can exert control on the community structure (RaE~MAN, KtnPERS, NOORDELOOS and WI~E, 1993). Similarly loss by sedimentation is commonly observed during bloom conditions and lysis, even in the absence of grazers, is conceivable, in which viruses may play a role (BRATBAK, EC~E and HELDAL, 1993).…”

Role of microbes in pelagic food webs: A revised concept

“…2). Strong supporting evidence for this concept of the structure of pelagic food webs comes from several recent reports of significant grazing, either postulated or directly measured, of metazooplankton, especially microcrustaceans, on phagotrophic flagellates and ciliates in various marine and freshwater systems (Gifford 1986;Sheldon et al 1986;Frost 1987;Gifford and Dagg 1987;Sanders and Porter 1987;Stoecker and Egloff 1987;Stoecker et al 1987a,c;Roman et al 1988;Turner et al 1988). …”