Microzooplankton herbivory and phytoplankton growth in the northwestern Sargasso Sea

Microzooplankton grazing and phytoplankton growth rates were measured with the dilution technique during spring, summer and fall in the oligotrophic Sargasso Sea near Bermuda. Phytoplankton growth rates estimated from changes in chlorophyll a ranged from 0.2 to 0.8 d-' and grazing rates from undetectable to 0.8 d-'. Grazing rates approximately balanced or exceeded growth rates, except on 1 date when no grazing was detected. The results support the view that phytoplankton in the subtropical oligotrophic gyres are growing at high, but sub-maximal, rates and are grazed to low stable blomass levels. Chlorophyll grazing rates in the surface mixed layer could be related to ambient chlorophyll levels by a Michaelis-Menten model with a grazing threshold. The close correspondence between the apparent feeding threshold level and the observed lower limit of chlorophyll concentration in the surface water of the Sargasso Sea suggests that the lower biomass limit may be related to threshold feeding behavior of the grazer community.