Phytoplankton growth and Zooplankton grazing in oligotrophic oceans

Abstract

Central oceanic regions such as the Sargasso Sea and the North Pacific Gyre have traditionally been thought of as biological deserts1,2. New phytoplankton production estimates3 and interpretations of physiological data4,5 have suggested that these areas are, in fact, highly productive and that nutrient exchanges supporting these high growth rates are extremely transient. Fast phytoplankton growth rates must be balanced by equally fast zooplankton-caused mortality. Zooplankton abundances and their filtration rates, however, are consistent with slow phytoplankton growth rates, not fast. It is shown here that the dominant grazers in oligotrophic areas seem to be microzooplankton, and that molecular diffusion limits the effect of nutrient pulses, implying that bulk nutrient concentrations are most important. Present growth studies imply that phytoplankton can meet their growth needs with nanomolar ammonia concentrations.