Phytoplankton activity in an oligotrophic environment was studied on six cruises over a 14-month period. Phytoplankton biomass and productivity displayed considerable temporal variability despite the relative constancy of the physical and chemical environment. No evidence of seasonality or diurnal variability in phytoplankton biomass was observed. Annual average (+ s.d.) depth-integrated values (0-260 m) for chlorophyll a, phaeopigment, ATP, and primary productivity were 24.55 + 10.31 mg {center_dot} m{sup -2}, 11.81 + 7.20 mg {center_dot} m{sup -2}, 3.00 + 1.78 mg {center_dot} m{sup -2}, and 8.79 + 7.82 mg C {center_dot} m{sup -2}, h{sup -1}, respectively; over the year these parameters were seen to vary over ranges of 3X, 6X, 10X, and 26X, respectively. The mean depths of the chlorophyll and phaeopigment maxima were 85 + 9 m and 95 + 11 m, respectively; the pheopigment maximum was always located at or below that of chlorophyll. Size fractionation studies showed that at this oceanic station about 80% of the phytoplankton biomass occurred in the < 5 {micro}m fraction. Low ambient nutrient levels were typical at the depth of the chlorophyll maximum, indicating that nutrient assimilation was actively occurring in that layer. Elevated nutrient levels were typical at the deeper phaeopigment maximum …